Reconstructing Holt Castle, Denbighshire

Little physically now remains of Holt Castle, however, one of the first projects the Castle Studies Trust was for leading buildings reconstruction artist Chris Jones-Jenkins and the late and much missed Dr Rick Turner to digitally reconstruct the castle using a mixture of historical sources and recent archaeological excavations to bring the castle back to life with a series of images and a video fly through. Here, in a piece first published in History Extra in 2015 explains how it was done.

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History:

To secure his conquest of north-east Wales, Edward I created five new Marcher lordships in 1282, and gave them as reward to his political allies and close friends in recognition of their loyal service. The lordship of Bromfield and Yale was granted to John de Warenne, sixth earl of Surrey (1231-1304). It is not known when he started building Castrum Leonis or ‘Chastellion’ as Holt Castle was known in the Middle Ages. The castle is first referred to in 1311 some years after it was probably completed. Warenne’s master mason chose an open and relatively flat site above the west bank of the River Dee, close to one of the ferry crossings into Cheshire. This allowed him to create a symmetrical pentagonal castle with a tall round tower at each corner, and the main rooms ranged around an inner courtyard. There were up to three storeys raised above the original ground level, with the moat and up to additional three basement levels cut down into the sandstone bedrock. It was a revolutionary design and was one of a group of similar architectural experiments being undertaken by the other new Marcher Lords at castles such as Denbigh, Ruthin and Chirk. At all these sites geometrical complexity, architectural grandeur and the provision of well-appointed accommodation seemed to be more important than their defensive capabilities.

Holt Castle as it was in its pomp courtesy of Chris Jones Jenkins

            The sixth earl was succeeded by his grandson John, the seventh earl of Surrey (1286-1347). Described by Alison Weir as ‘a nasty, brutal man with scarcely one redeemable quality’, he was involved with the political upheavals of Edward II’s reign, including the loss of Holt Castle for a time to Thomas, earl of Lancaster. This John died without a legitimate heir, and Edward III intervened to settle his estates upon Richard FitzAlan, third earl of Arundel (c.1313-1376). His son, Richard the fourth earl (1346-1397) was one of the ‘Lords Appellant’, who had curbed Richard II’s powers and had the king’s closest allies executed or exiled during the ‘Merciless Parliament’ of 1388. It took nine years before the king was able to wreak his revenge on these powerful men. Arundel was brought to trial for treason at Westminster on 21 September 1397, and was convicted and beheaded on the same day. Richard II seized his estates and incorporated Bromfield and Yale into his new principality of Cheshire, with Holt Castle as its main stronghold. Building work was commissioned in 1398, which saw the Chapel Tower extended and included a water-gate, called ‘Pottrell’s Pit’ in later documents, linked to the river. Over the next year more than £40,000 in coin, jewellery, gold and silver plate was transferred from the royal treasury in London to Holt for safe keeping. When Henry Bolingbroke – later Henry IV – returned to England in 1399, he shadowed Richard II up the Welsh Marches and was quick to recapture Holt Castle. Despite being defended by 100 men-at-arms and being well provisioned, Henry’s men, including the French chronicler Jean Creton, were able to enter through the new water-gate and ascend ‘on foot, step by step’, to take the castle unopposed and so recover this vast proportion of the king’s disposable wealth.

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            Over the next 139 years, three more owners or stewards of the lordship of Bromfield and Yale were to be executed for treason. The first was Henry Stafford, second duke of Buckingham (1455-83). A year after his death, Sir William Stanley (c.1435-1495) acquired the lordship from Richard III. Despite his role at the Battle of Bosworth which brought his relative Henry VII to the throne, he was implicated in the Perkin Warbeck affair and was executed in 1495 after which Holt Castle reverted to the Crown. The last of this group was William Brereton (c.1487×90-1536) who was appointed steward of the castle where he held ‘great porte and solemnities’, before he was convicted of adultery with Anne Boleyn and beheaded.

            Holt Castle remained the property of the Crown and was to form part of the estates of the Princes of Wales. It was held for the king during the Civil War and in the 1670s, the buildings were sold off to Sir Thomas Grosvenor, who systematically dismantled nearly all the fabric and transported it to help in the building of his Eaton Hall, a few miles south of Chester. This has left us with the stump of rock and walling which survives today.

Reconstructing the Castle:

Two attempts have been made to understand the original the former appearance of Holt Castle: by Alfred Palmer in 1907 and Lawrence Butler in 1987. They had access to plans and views of the castle drawn in 1562 and 1620. The problem they faced was that these two sets of drawings are contradictory and they were hard to relate to what survived. Since 1987, new evidence has been made available:

  1. Documentary evidence for Richard II’s building work.
  2. The publication of a transcript of the Holt Castle inventory taken after Sir William Stanley’s arrest in 1495.
  3. The discovery of a new early-seventeenth century plan of the castle in the National Library of Wales.
  4. A programme of masonry consolidation, archaeological excavation and survey led by Steve Grenter, Wrexham County Borough Council, in partnership with the Holt Local History Society.

This new work prompted me to undertake another attempt to reconstruct Holt Castle. With the help of a grant from the Castle Studies Trust, I was able to commission the well-known castle reconstruction artist, Chris Jones-Jenkins, to develop a 3D digital model which was flexible enough to capture and assimilate the new data and modify the structure as new insights were gained.

The challenge has been to weigh each source of evidence and identify which strands should have precedence when contradictions emerge. A hierarchy was developed:

  1. The visible masonry of the castle and the modern topographical survey of the site provided the primary terrain model on which the remainder of the castle was constructed.
  2. The archaeological evidence for the rock-cut footings, the bases of three of the towers and the line of the channel leading into the water-gate provided other fixed points in the model.
  3. By considering the content and the potential reasons for the drawing of the three, early-modern ground plans and the two associated views, that of 1562 was assessed to be the most reliable for the details that it showed, the early seventeenth century plan for its dimensions, and those by John Norden of 1620 to be the most inaccurate, despite them being the ones most frequently published.
  4. The list of rooms and the route followed by the appraisers of the 1495 inventory provides the most comprehensive source for the internal layout of the castle. However, it became clear that those rooms without contents, such as the triangular ante-rooms to the towers and the latrines, were not listed.
  5. None of the views of the castle were architecturally detailed, and none showed all of the exterior or much of the inner courtyard. This information had to be derived from surviving details from other Edwardian castles in North Wales or from work undertaken in Richard II’s reign, when considering the projection from the Chapel Tower.
Holt First floor floor plan with with watergate to the fore and great hall and stable section to the right, copyright Chris Jones-Jenkins

The development of the model became an iterative process between the historian and the artist. Different combinations of layout and floor heights were tried, building on the fixed points surviving at basement level and rising up in an effort to accommodate all the rooms. Suites of accommodation began to emerge. Sir William Stanley’s great chamber led off the high end of the hall and controlled access in one direction to his counting house and the chapel, and in the other to the Treasure House and the High Wardrobe, where all the valuables were kept. Across the courtyard his wife, Elizabeth, had her own great and bed chamber, linked to the nursery. Above her accommodation was a chamber for her gentle women, well away from the yeomen’s dormitory under the lord’s great chamber. One range was dominated by the kitchen, rising the full height of the castle, connected at basement level to a larder, pastry house, well house and a wine cellar on one side and offices for the cook and butler on the other. As was normal the constable had a suite over the inner gate, but unusually a stable for 20 horses was created in a basement below the hall with access for the animals across the moat and up a ramp into the castle.

Holt Great Hall and Stables copyright Chris Jones-Jenkins

Working on this reconstruction, our admiration only grew for the original designer. He created a symmetrical plan and external appearance but produced complex internal arrangements to meet his patron’s needs. Holt is as much a chivalric ideal as a practical castle. An animation of the reconstruction of Holt Castle has been produced by Mint Motion of Cardiff and can be viewed here:

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Feature image courtesy of Wrexham Borough Council

Haverfordwest’s town wall revealed?

Neil Ludlow and Phil Poucher of DAT look at the results of the investigation at Haverfordwest Castle by Dyfed Archaeological Trust (DAT), as part of a major infrastructure scheme embracing the castle and its setting, has revealed what may be part of the medieval town wall, long thought to have been entirely destroyed.

The remains of the castle still dominate views of the town, particularly from the main eastern approach, crowning a steep bluff overlooking the Western Cleddau river. Founded around 1110 by one Tancard, a Flemish colonist, the castle appears to have begun as a partial ringwork and bailey, perhaps adapted from an Iron Age hillfort. Fortification in stone began under Tancard’s grandson Robert FitzRichard, a decade or so either side of 1200, with the erection of a subrectangular donjon; a curtain wall with at least one round mural tower was later added, possibly by the younger Marshal earls of Pembroke between 1219 and 1245. The castle was transformed into a palatial residence with the addition of an integrated suite of apartments of the highest quality, including hall and chamber-block ranges, and a terraced garden enclosure; they are traditionally attributed to Edward I’s queen Eleanor of Castile who received the castle and lordship two years before her death in 1290. The outer ward was also walled in stone, probably during the early fourteenth century. Although it played no part in the second Civil War of 1648, the castle was partially slighted on Cromwell’s orders and was subsequently used as a gaol, which closed in 1878.

Open to the public since 1970, and housing the town’s museum and County Record Office – but still perhaps an under-valued asset – the castle is now the subject of an enhancement programme to improve access, carry out essential repairs and redevelop the museum. The scheme extends to the castle’s setting, with improved landscaping and restoration of the surrounding burgage-plot boundaries. Preliminary archaeological work includes geophysical survey and test-pit recording.

Figure 1: The walling and archway from northeast. The castle donjon is at far left.

Investigating the castle exterior in early 2021, at the summit of the steep bluff, Andy Shobbrook of DAT came upon a stretch of walling that appears to have evaded previous investigations. Now of no great height, but probably truncated, it is pierced by a wide segmental arch of convincingly medieval form (Fig. 1). Although absent from published plans and descriptions of the castle, it is shown on the large-scale 1:500 map of the town produced by the Ordnance Survey in 1889, on which it is labelled ‘Arch’ in the Gothic script reserved for antiquities (Fig. 3). It lies just within the scheduled area of the castle, corresponding with its boundary, and appears to be in a stable condition.

Figure 2: Plan showing the conjectured layout of Haverfordwest in c.1300

The walling may be part of the medieval town wall rather than the castle defences. The town of Haverfordwest, which is notable for its three medieval parish churches – unique in Wales – was founded soon after the castle and by the close of the Middle Ages had become the de facto county town of Pembrokeshire. Defended by an earthen bank and ditch from an early period, probably before 1200, it was walled in stone after the issue of a murage grant in 1264. The defended area was relatively small, immediately next to the castle and always known as the ‘Castleton’ – while the extensive suburb around the extra-mural marketplace to the south received fortified gateways, they were never connected by any solid barrier (Fig. 2). The town wall had largely disappeared by 1700 and, while the gatehouses survived rather longer, the last were removed at the end of the eighteenth century.

Figure 3: An extract of the Ordnance Survey 1:500 map of Haverfordwest, of 1889,
showing the castle and walling (labelled ‘Arch’).

Vestiges of the wall were apparently still detectable in 1900 but all traces were thought to have been lost soon afterwards. Stretches of its former line are marked by property boundaries but its entire course is not precisely known, nor the points at which it connected to the castle defences. The walling discovered in 2021 butts against the donjon at the northeast corner of the castle inner ward, and runs northwest for 5 metres before petering out. The remains of a return at its northwest end correspond with a 90° turn shown on the 1889 map, on which it is shown to then run north-eastwards before turning west to continue along the outer edge of the castle’s northern ditch. But the medieval wall must have deviated from this line at some point, to run northwards to the eastern town gate. The arch is 3 metres wide but was probably always too low – and perhaps too wide – to represent an entry. Its function may simply have been to drain the area immediately to the west, which slopes steeply downhill towards the east and seems to have been a continuation of the castle ditch where it ran out at the crest of the bluff (Fig. 3). Two phases of work within the arch are possible, suggesting it was modified and perhaps narrowed at some point.

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Dún an Óir Castle: an uncertain future

Dr Sarah Kerr of Trinity College Dublin discusses the ongoing project to look at the impact of climate change on castles with a focus on the ones in West Cork, as they are battered by Atlantic storms.

Over the past two years, two small grants (Research Incentive Scheme, Trinity College Dublin, Ireland; and Higher Education Innovation Fund, UK) has funded research into the impact of climate change on built heritages, and the identification of those most vulnerable, particularly in West Cork, in the south-west of Ireland. Dún an Óir is one such castle at risk, on the edge of Ireland, on the brink of the West Cork cliffs and at the mercy of the increasingly frequent Atlantic storms and high winds.

Dún an Óir is a castle at risk, on the edge of Ireland, on the brink of the West Cork cliffs and at the mercy of the increasingly frequent Atlantic storms and high winds. Dún an Óir is an Irish tower house, a type of stone-built castle, smaller than the sprawling castles built by the Anglo-Normans, such as Trim, County Meath and, indeed, serving a different purpose. The relative small size of tower houses placed their construction within the financial reach of lords, emerging gentry and merchants in both rural and urban areas. Tower house construction commenced on the island of Ireland (Ireland hereafter) in the mid-fourteenth century and flourished in the fifteenth and sixteenth, construction ceasing by 1650. Their abundance led Terry Barry to state that Ireland was ‘the most castellated country in Europe in the late middle ages’ (Barry 2008, 129) or at least ‘one of the most castellated parts’ of the Irish and British Isles (Cairns 1987, 31; O’Connor 1998, 25). If so, then Munster in the south-west of Ireland was a focal point of this activity. The West Cork area in the south-western extent of Munster contains 47-known tower houses, many concentrated along the jagged and island-dotted Atlantic coast. The West Cork tower houses are of particular interest as they are the only castle type remaining in the region; plus, contemporary written evidence allows the majority to be ascribed to a certain clan.

Dún an Óir (Doonanore when anglicised) is located on the island Oileán Chléire (Clear Island), approximately 13km from the Irish mainland. It was built by the O’Driscoll clan in the mid-sixteenth on a coastal promontory (Figure 1). Located on the coast, along with several other known O’Driscoll tower houses, its occupants could oversee the movement of goods through Roaringwater Bay as well as charge passing vessels for anchorage, victualing and exploitation of the waters (Figure1). 

This once strategic position is Dún an Óir’s Achilles heel. The building and its immediate landscape are precarious, unconsolidated and unprotected (Figure 1). Exposed on a partially collapsed promontory, its long-term survival is unlikely and our time to understand it short.

Figure 1: Photograph looking north across Roaringwater Bay towards the Atlantic showing Dún an Óir on a rocky promontory.

Dún an Óir comprised four-storeys rising from a rectangular ground-plan. Access was through the south wall, although the east wall faced the neck of the promontory (where it connected to the remainder of the island). Much of the south wall has collapsed yet a garderobe tower remains to the east of the door (Figure 2). Above the doorway a mural staircase (built within the thickness of the wall) can be seen leading to at least the first and second floors. There is a vaulted ceiling between the second and third floors in the form of three pointed arches separated from one another by overlapping slabs.

Figure 2: Dún an Óir tower house, from the southeast, showing the mural staircase extending upwards along the south wall (with thanks to Finola Finlay and Robert Harris for the photograph).

A significant and surviving feature of Dún an Óir is its bawn (curtain wall): a stone wall which in this case abuts the tower house (rather than surrounding it) and stretches west to enclose the remainder of the promontory (Figure 3). Vicky McAlister’s recent book on tower houses indicates that the survival rate of bawns is low compared to the tower houses themselves (McAlister 2019, 22), recalling earlier suggestions that the current 1 in 5 survival rate may be representative of their former inclusion. Dún an Óir is a relatively rare example of preserved ancillary buildings within the bawn. Although they remain only as overgrown footings, there appears to be a kitchen with oven, and two more apparently feature-less buildings that appear to be contemporaneous with the bawn wall (Figure 3).

Figure 3: A simple plan of Dún an Oir tower house, bawn, and ancilliary buildings. The proximity tit he promontory edge is clear.

Dún an Oir’s bawn reaches to the edge of promontory, almost certainly to be further damaged or lost in this lifetime. With this seemingly fixed future, the onus shifts to the researching community in the present to document what we can: this is discussed further in a recent paper on the castle (Kerr 2019). The erosion to the promontory renders further buildings survey, as well as geophysical survey or excavation, no longer safe. Therefore, the next steps of fieldwork may include airborne LiDAR. Greater exploitation of innovative surveying methods may overcome the difficulties posed by the physical landscape and allow the tower house, bawn and ancillary buildings to be understood to a greater depth before the inevitable happens.

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Barry, T. 2008. The study of medieval Irish castle: a bibliographic survey. Proceedings of the Royal Irish Academy: Archaeology, Culture, History, Literature 108(C), 115–36.

Cairns, C. 1987. Irish tower houses: a Co. Tipperary case study, Group for the Study of Irish Historic Settlement.

Kerr, S. 2019. Reconnecting Cultural Landscapes: Dún an Óir, West Cork, Ireland, Landscapes, 20:2, 160-177, DOI: 10.1080/14662035.2020.1861725

McAlister, V. 2019. The Irish tower house, Manchester University Press.

O’Conor, K. 1998. The archaeology of medieval rural Settlement in Ireland, Discovery Programme Monographs.

Barns in the Bailey: Agricultural buildings within castles

Castles were more than military sites, being political, judicial and economic centres too. Duncan Berryman of Queen’s University Belfast looks at one of those aspects, the important but little studied area of agricultural buildings in castles.

The domestic considerations of castles were as important to daily life as their defensive functions. Castles were also the centres of estates; Clare Castle (Suffolk) is a good example of an estate centre with manors in the surrounding region and further afield while Caister Castle (Norfolk) was the centre of a much more compact estate. Manorial centres were generally undefended courtyards of domestic and agricultural buildings. However, many of these were moated sites and their lack of defensibility might be debated. Sites such as Chalgrove (Oxfordshire) and Coolamurry (Wexford) had substantial moats and may have had complex bridges (Page et al. 2005; Fegan 2009). These complexes saw high-status accommodation sitting alongside the barns and animal housing that served as the manorial farmyard. Many smaller castles would have been the centres of smaller estates and would have also functioned as manorial centres. Agricultural buildings were vital to this operation and would have been found within baileys or associated enclosures. To investigate these buildings, it is essential to take a multidisciplinary approach by combining archaeology with documentary and pictorial evidence. The agricultural buildings have often been left unexcavated as they are seen as less important, and less interesting, than the domestic complexes.

Artistic reconstruction of Clough Castle (Down). Many motte and baileys would have had a similar appearance, often with timber structures on the motte instead of the masonry buildings presented here.

Documentary sources related to Ireland indicate that many smaller castle sites had agricultural buildings within the surrounding area. This is clearly illustrated at Cloncurry (Kildare), where an extent records that beside the motte was “an old haggard close [farmyard] in which there are two small granges [barns] each of eight forks … [t]here is also there, beside the gate an ox-house … [and t]here is a dovecote” (Murphy & Potterton 2010, 175). There is also a record of a barn of ten forks at Castlemore (Carlow), as well as a mill and other timber and masonry buildings (O’Conor 1998, 32). The forks referred to in these accounts are probably the cruck blades of the barn, thus ten forks produce a nine-bay barn that could be the equivalent size of a barn like Bredon (Worcestershire). At Lough Merans (Kilkenny), the motte sat on a promontory in the lake and had an associated bailey containing a granary, stables, and a sheepcote (O’Conor 1998, 30). The motte of Inch (Tippararery) also had a granary and sheepcote, additional buildings were stables, a fish-house, a dovecote, and a malt kiln (O’Conor 998, 29). These examples suggest that, in Ireland, castles were playing important roles as the centres of manorial farms and wider estates. The perceived need for a defensive motte may have been carried over from an earlier period, but it continued with the construction of tower houses in the fifteenth and sixteenth centuries (McAlister 2019, 28-35).

Bredon Barn (Worcestershire). Barns of a similar size were found within castle baileys. They would have been constructed from masonry or timber

A brief survey of larger castles in England does show that these castles had a small number of agricultural buildings. The main buildings identified were stables, dovecotes, barns, and granaries. These buildings are unlikely to have functioned in the same way as they did in rural manorial centres, but they are a reminder that some agricultural processes occurred within the castle. Stables were obviously for housing the riding horses of the lord or king and their entourage, but they probably also housed the cart horses used by the castle staff to bring goods in from the market. There are records of stables at Hereford, Leeds (Kent), Leicester, Lincoln, Nottingham, and the stables of Kenilworth (Warwickshire) are still extant (Colvin 1963). The dovecote was an important source of food in the form of squabs (young pigeons). A dovecote was recorded at Dorchester (Dorset) and one has been identified in the south-west tower of Bodiam (Sussex) (Colvin 1963; Thackray 2003). The existence of barns might suggest that sheaves of wheat or barley were being brought into the castle and threshed there, providing grain for flour and straw for the stables and other floors around the castle. There is evidence for barns at Acton Burnell (Shropshire), Bamburgh (Northumbria), Hadleigh (Essex), Weoley (Warwickshire), and Pontefract (Yorkshire) (Colvin 1963; Emery 2000; Roberts 2002).

The stables of Kenilworth castle (Warwickshire). This was a particularly grand building, but stables were vital for the lordly lifestyle.

The c.1610 plan of Castle Hedingham (Essex) indicates that there may have been an agricultural complex beside the main castle enclosure (Emery 2000, 113). To the top of the map are buildings labelled stable and barn and an area marked barnyard, in manorial sites this would indicate a complex of crop storage buildings including several barns and a granary. Returning to Clare Castle, the 1325 accounts of the constable indicate that there were a pigsty and poultry house within the castle compound (Ward 2014, 64).

Agricultural buildings are found in areas away from the main domestic complex of the castle, and these areas are often left unexcavated. They were most often made of timber, and thus leave little trace in the archaeology compared to the stone-built structures elsewhere. It is possible that there were more agricultural buildings within castles, or close to them, and they have not yet been identified. This research highlights the importance of using documentary and pictorial sources alongside archaeology and reminds us that lesser castle complexes may have been surrounded by a significant range of buildings.

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References

Colvin, H.M. (ed.) 1963, The History of the King’s Works Vol I & II (London: Her Majesty’s Stationary Office).

Emery, A. 2000, Greater Medieval Houses of England and Wales II (Cambridge: Cambridge University Press).

Fegan, G. 2009, ‘Discovery and excavation of a medieval moated site at Coolamurry, C. Wexford’, in C. Corlett & M. Potterton (eds), Rural Settlement in Medieval Ireland in the Light of Recent Archaeological Excavations (Dublin: Wordwell Books).

McAlister, V. 2019, The Irish Tower House: Society, Economy and Environment c.1300–1650 (Manchester: Manchester University Press).

Murphy, M. & Potterton, M. 2010, The Dublin Region in the Middle Ages: Settlement, Land-Use and Economy (Dublin: Four Courts Press).

O’Conor, K.D. 1998, The Archaeology of Medieval Rural Settlement in Ireland (Dublin: The Discovery Programme).

Page, P., Atherton, K. & Hardy, A. 2005, Barentin’s Manor: Excavations of the moated manor at Harding’s Field, Chalgrove, Oxfordshire 1976–9 (Oxford: Oxford Archaeology Unit).

Roberts, I. 2002, Pontefract Castle: Archaeological Excavations 1982–86 (York: West Yorkshire Archaeology Service).

Thackray, D. 2003, Bodiam Castle (Swindon: The National Trust).

Men on the Watch: The Motte d’Olivet, Grimbosq, Normandy

In her new book Authority, Gender and Space in the Anglo-Norman World, 900-1200, Dr Katherine Weikert, Senior Lecturer in Early Medieval European History at the University of Winchester looks at how medieval manors reflected and shaped – and were shaped by – their occupants to express social authority. In this article she looks at the facinating if little known Motte d’Olivet.

High in the lush forest of Grimbosq, Normandy, the remains of the Motte d’Olivet sit atop a lofty spur overlooking the Orne River valley. About twenty-two kilometres to the south of Caen, it’s a site that would be familiar to many castle-lovers: a man-made earthen mound, atop which formerly stood a wooden tower, with baileys on either side of it. A deep ditch around the motte kept it spatially and physically separated from both baileys. The motte-and-baileys take up the cramped, flat space on the point of the v-shaped spur surrounded by steep drops and small rivers on both sides (one charmingly called Ruisseau de Coupe-Gorge: essentially, cut-throat creek). The tower on the motte overlooked the whole area, and when the brush and forest was cut down around it, as it was in the eleventh century, the viewshed would have been spectacular. The Motte d’Olivet has long been considered a seigneurial residence, but its location, and the history of the man who built it, suggest a far less lofty purpose.

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Image 1: Plan of the site of the Motte d’Olivet. Pighill Archaeological Illustrations redrawn from Decaëns 1981, published in Weikert 2020.
Image 2: View north from the motte towards the River Orne. The stone remains in the foreground are suggested to be part of a gatehouse to access the motte, and the modern posts are placed at the location of the posts of an aisled building. Photo by the author.

The site was excavated by Joseph Decaëns and published in 1981, though lingering questions remain about the castle and its builder. The elusive Erneis Taissan built the motte in the 1040s-50s during a period of conflict with his older brother Raoul. The Taissons come to greater prominence in Normandy in the twelfth and thirteenth centuries along Raoul’s line; Ralph Taisson, for example, was King John’s seneschal in Normandy at the turn of the thirteenth century. But in this earlier period, the family is more difficult to uncover, and the remains of the Motte d’Olivet are some of the only physical presence we have of them from this time. Decaëns even described Raoul as ‘un personnage très important mais assez énigmatique’ (a very important, but very enigmatic person)!

Seeking evidence of the brothers and their history is an exercise of charters, patience and this castle. Decaëns ascribed the tension between the brothers to each receiving portions of their father’s lands after his death, and Norman charters indicate some expression of this frustration through the brothers’ conflicting patronage of local monasteries. For example, the elder Raoul founded the abbey at Fontenay (St-Étienne de Fontenay, on the Orne north of Grimbosq) in 1050, and its foundation charter, confirmed by William the Conqueror, lists properties Raoul gave to the abbey, all to the east of the Orne. Erneis – who patronised Fontenay but may have preferred the abbey at Fontenelle – held estates mostly to the west of Raoul’s. Grimbosq and Cesny(-Bois-Halbout) are but two of these, with his descendants’ title even named for them. The lines drawn between the properties of these two brothers run straight through the forest where Erneis built his Motte.

Image 3: The remains of St-Étienne de Fontenay, outside of St André-sur-Orne, Normandy, founded by Raoul Taisson c. 1050. Photo by the author.

Regardless of the cause of the tension, the Motte is the most obvious indication of the brothers’ strife, which visibly plays out the Norman landscape. The Motte directly overlooks the boundaries and roads between Grimbosq – Erneis’ land – and Mutrécy – Raoul’s land – with a clear sightline to the valley to the north, and a ridge to its east leading to Mutrécy. With this in mind, alongside the size of the castle and particularly its distance and isolation from any settlement, the Motte looks less like a seigneurial castle and more like an outpost, a short-lived watchtower to keep an eye on one’s pesky brother. It’s deeply unlikely this motte was Erneis’ main residence, but instead a place for his men to watch the boundaries of his land.

Image 4: The top of the motte, Mutrécy over the horizon. Photo by the author.

The northern enclosure holds a large aisled building, possibly two-storeyed, and additional stone rooms or buildings which have been interpreted as a sort of a gatehouse providing access to the motte, and a small separate kitchen. The apsidal room was previously interpreted as a chapel but this has recently been rejected by me on varying grounds (including location, orientation and charter evidence), which further indicates this place as less an elite residence and more an outpost. However, the state of the physical remains makes it unclear how to interpret not only the décor but quite simply the form and use of these buildings. The north bailey would probably have housed Erneis when he was there. But if that was this bailey’s primary intention, it would have only occasionally seen habitation, and more frequently seen the men on the watch accessing the motte via the gatehouse. The intention of this castle wasn’t in residency, but in the duties it performed for Erneis, in the watching and waiting done by his men.

Image 5: Remains of the ‘kitchen’ in stone, the modern post-holes marking the location of the large aisled building in the north bailey, with the motte (covered in scrub) in the background. Image by the author.

The castle’s activity would have been focused in the south bailey. The south enclosure, which held a stable and forge, would have hosted a number of people and animals, and alive with the sights, sounds and smells of a working yard – hot iron and manure, the hammer singing against the anvil. It would have been a site of activity at the castle, a place for the retinue who staffed this border outpost. Game pieces found in excavation mean men entertaining themselves, or perhaps simply passing the time, the way you’d play cards in an airport. As Susan Reynolds and many others have pointed out across the middle ages, military men of a sort gathered together to wait and watch would have probably had some training or practice to attend to themselves: we can envision the south enclosure for this.

Image 6: The south bailey from the top of the motte. Note the deep ditch surrounding the motte. Image by the author.

Wace tells us that the sons of the two men, Robert fitzErneis and Raoul (II) Taisson, both died at Hastings fighting with Duke William, and the rift between their two families was only repaired at this point. Erneis and Raoul (I) were probably already deceased and, to wit, the Motte was no longer in use by the FitzErneis line. Neither the Motte d’Olivet nor the warring brothers have been further investigated since the site’s excavation in 1981, and new questions about the place and its people can bring different views about what was once considered an elite house. What’s left of the brothers’ rift is just the landscape of this ruined castle, a lonely enclosure in an isolated place, high above the flourishing green forest, a lesson reminding us that castles have much to say about all the people who were once there.

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Katherine Weikert’s new book, Authority, Gender and Space in the Anglo-Norman World, 900-1200, is available from Boydell & Brewer at http://boybrew.co/2YyHx2m. Use the special discount code BB870 for 40% off and free international shipping!

Grimbosq Forest is a part of the city of Caen. It is open to the public and filled with walking and mountain cycling routes, and sites such as an arboretum, a pet cemetery, and the Motte d’Olivet. The Motte is on the yellow walking route, and is unstaffed thought signage gives information about the site. There is free parking, and the forest is also on bus route 34 from Caen. More information can be found at https://caen.fr/annuaire-equipement/foret-de-grimbosq (French) or https://www.calvados-tourisme.co.uk/offer/la-motte-castrale-dolivet/ (English).

Castle Studies Trust 2021 Grant Awards

The Castle Studies Trust is delighted to announce the award of six grants, totalling a record £31,000 not only covering a wide geographic area but also a wide range of different types of research:

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Caerlaverock new castle with old behind it and the coastline Crown Copyright Historic Environment Scotland

Caerlaverock, Dumfriesshire

The aim is to understand the chronology and geography of extreme weather events in the high medieval period, and the effects they wrought on archaeological features that led to the abandonment of the old castle built in c.1229 in favour of the new built 200m away in c.1277. The latest thinking is that it was a series of extraordinary storm surge events which pushed a series of storm driven gravel ridges across the River Nith.

The methodology to find this out is interdisciplinary, using scientific methods to enhance understanding of archaeological fieldwork. The fieldwork will involve the establishment of a series of transects across the site and surrounding landscape from which cores and samples will be extracted for sediment description, stratigraphic analysis, and Carbon 14 dating.

Depending on Covid restrictions, the aim is to start doing the work in May this year with the receipt of the final data in the autumn.

Greasley_Castle from air copyright Neil Gabriel

Greasley, Nottinghamshire

The production of an interpretative phased floor plan for Greasley Castle in Nottinghamshire.  The castle, built in the 1340s, has an obscure history and the understanding of its architectural phasing is at best very cloudy. The site is now a working farm and a number of post-mediaeval structures have been conglomerated around the remains of what is suspected to be a fourteenth century courtyard house with projecting corner towers.

The survey will act as baseline research data for a site which has not previously received serious fieldwork or publication and provide a basis for further research but also for any future conservation needs.

Work on the project will start in the early summer when covid restrictions ease.   

Laughton-en-le-Morthen motte and bailey castle and church

Laughton-en-le-Morthen, South Yorkshire

To provide professional illustration and reconstruction which will also be integrated into a co-authored academic article based on the two previous research projects carried out on the site by Dr Duncan Wright and funded by the Trust. A geophysical survey and then small-scale excavation which give a strong indication that the Normans had built a motte on the site of a high-status Saxon dwelling.

Part of the monies will be used to produce phase plans of Laughton during key stages of its development, and a small percentage will pay for a line drawing of the 11th century grave cover incorporated into the fabric of the nearby church. The aim will be to start the work as soon as possible.

Old Wick Tower copyright Historic Environment Scotland

Old Wick, Caithness

Dendrochronological assessment of timber at the Castle of Old Wick, Caithness thought to be one of the earliest stone castles in Scotland dating from the12th century and the period of Scandinavian ascendency. Current thinking though ascribes the date to the 14th century. Analysing these samples will hopefully provide an answer.

With no architectural features or physical “independent” evidence analysing the remains of a timber joist-end (in poor condition) in one of the joist ends remains the best chance of being able to find an answer. 

The taking of the samples is likely to take place in September when conditions are still going to be favourable as the castle is situated next to the North Sea and the sample can only be found 8 metres above ground level. 

Richmond Castle copyright English Heritage

Richmond, North Yorkshire

Co-funding a three-week excavation of Richmond Castle, one of the best preserved and least understood Norman castles in the UK. The aim is to understand better the remains of buildings and structures primarily along the eastern side of the bailey including near the 11th century Robin Hood tower and near Scolland’s Hall.

Subject to the scheduled monument consent being granted the excavation will take place in late July.

Warkworth Castle, copyright William Wyeth

Warkworth, Northumberland

Geophysical survey to explore evidence for subsurface features in and around the field called St John’s Close in a field adjacent to the castle with the aim to establish the location and eastern extent of the castle’s deer park in the 16th century as well as its entrance way. It also hoped to find evidence of a routeway running parallel to the possible park boundary which could represent an early route to the castle’s gatehouse from the south-west.

The plan is to do complete the geophysical survey by the end of March.

To keep up to date with how these projects progress over the coming months you can:

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And if you donate at least £50 here and be invited to our exclusive visits to these projects: https://donate.kindlink.com/castle-studies-trust/2245 

Featured image: Old Wick Castle, Caithness, copyright Historic Environment Scotland

What lies beneath: what a ground penetrating radar survey reveals of Druminnor Castle

Lead Archaeologist on the Druminnor Castle excavation, Dr Colin Shepherd, looks at the results of the ground penetrating radar (GPR) survey of the castle in funded by the Castle Studies Trust in 2019 and what that means for future work at the site.

As a consequence of the GPR geophysics, that were generously funded by the Castle Studies Trust, we have a number of new and potentially interesting anomalies to be investigated. Furthermore, the GPR research has also been instrumental in clarifying other aspects of previously excavated evidence. This has been made possible as, contrary to most instances, the geophysics at Druminnor have been incorporated into the ongoing investigation rather than simply as a precursor to excavation.

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The GPR generated 286 ‘radargrams’ or sections across the site and much time and effort has gone into analyzing these. The radargrams were compared to actual excavated sections. This permitted the creation of a ‘key’ that has allowed us to extrapolate from the excavated sections to unexcavated parts of the site. As a result, we can, fairly comfortably, suggest a developmental plan of Druminnor spanning the 13th to 18th centuries (Figure 1).

Suggested phasing of Druminnor Castle by Colin Shepherd

The GPR has suggested the extent of a hardcore platform that appears to have supported the ‘Old Tower’. This was thought to have been the earliest part of the castle but without hard evidence. We hope to test the presumed limit of this platform as we extend our trench southwards from the well that sat within the tower. (As you will recall, the GPR alerted us to that remarkable feature that became a focus for our 2019 season.) The GPR suggests that there may be a revetment supporting the platform at that point. Also, any relationship between this platform and the mid 12th-century kiln will be important in attempting to date that platform and, by extension, the Old Tower itself.

Well found in tower as a result of the GPR survey

The radargrams have also demonstrated the probable line of a terrace and sunken formal garden (red hachures on plan) and permits the excavated section of revetment and steps (shown in black) to be placed in its proper context. This garden probably dates to the 16th or 17th century, though exactly when is still debatable. The construction of the terrace removed all trace of the earlier (early to mid 15th century) ditch and so must post-date that. A late 17th-century Dutch pipe bowl found in the fill of a post hole associated with the steps suggests its removal at around that time or shortly thereafter. No trace of the terrace appears on the estate plans of c.1770.

The line of the defensive ditch has been shown by the radargrams to extend westwards from the excavated portion in Trench 11. This gives us an accurate alignment permitting its course to be drawn, even though the middle section was removed by the terrace. Where its return is at the western end of the site is still a mystery. The eastern arm was excavated in Trench 2 lying beneath the later (early 16th-century) lower courtyard. The 15th-century upper courtyard incorporated the tower with the surviving hall block (shown in blue). The courtyard’s west range can be estimated from remains found during the kiln excavation, but its eastern boundary was removed when the lower courtyard was added.

Finally, the GPR revealed the line of the outer enclosure wall as depicted on the estate plans. This was sampled in 2019 and the base of the wall found. The date of this feature is presently unknown and further work needs to be done to try to clarify that matter.

Evidence of landscaping material in outer enclosure copyright Iain Ralston

Sadly, work planned for 2020 included the extension of the ‘well trench’ in the car park to look for the possible revetment suggested by the GPR as well as opening a much bigger area across the outer enclosure wall, as located by GPR. This sits beneath a good metre and a half of later landscaping material (see picture, courtesy of Iain Ralston) and, for safety and archaeological reasons, demands a much larger trench. The keyhole sample trench indicates good survival and, it is hoped, evidence will be found to date the feature and to help explain when this became the northern boundary of the castle enclosure.

The excavated evidence together with the GPR results permit this developmental plan to be made and rough dates affixed. It is worth noting that, prior to our investigations, Druminnor was believed to have consisted solely of the surviving hall block with an attached tower at its western end. Everything not in blue on the plan, therefore, was formerly completely unknown.

Access in 2021 is still, sadly, in the laps of the Gods owing to Covid, but it is hoped that we will be able to get back on track later this year to look for more goodies!

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‘All Along the Watchtowers! Balancing Heritage Protection, Development, and Scientific Research on Buried Archaeology at European Castles’

Dr Rowena Banerjea, University of Reading, UK, with funding from a British Academy/Leverhulme Small Research Grant talks about the research project she works on All Along the Watchtowers! https://research.reading.ac.uk/castle-geoarchaeology-heritage/

The buried or below-ground archaeology at castle sites can be exceptionally well preserved because of its burial under masonry. However, it is often overlooked in conservation and management plans in favour of protection and consolidation of standing remains and developments such as reconstruction and rebuilding projects. It provides an important lens through which we can examine important phases and hiatuses in the developmental history of these monuments.

INSERT FIGURE 1. Well-preserved stratigraphy at Château de Gien, France, from c. 9th – 11th AD occupation, sealed below later masonry and collapse. The rectangles show where micromorphology samples were collected, and the circles mark the locations of spot samples for other scientific analyses. Photo credit Quentin Borderie.]

This project has been made possible due to its project partners and collaborations with several other research projects. The castle case studies represent a diverse range of cultural, environmental and heritage contexts across Europe, in which we could examine the preservation of stratigraphy, biological remains, artefacts and materials:

  • Elbląg, Poland, Karksi, Estonia, Cēsis, Latvia, important Teutonic Order castles in the Baltic region;
  • Castle Keverberg (Kessel), the Netherlands, which consisted of an original stone tower built in 1100 and covered by a motte-and-bailey castle;
  • Several French castles, which are Château de Caen, built in 1060, Château de Gien, which today hosts the National Museum of Hunting with the remains of an early (c. 9th – 11th AD) Carolingian castra below, Château de Loches and Château de Boves;
  • Two Italian case studies, Castelseprio, a UNESCO World Heritage site, and Montegrotto;
  • Molina de Aragón, Spain, situated in the Molina and Alto Tajo UNESCO Global Geopark;
  • Dunyvaig Castle, Islay, Scotland, formerly the naval fortress of the Lords of the Isles, the chiefs of the Clan MacDonald.

This project developed a framework for geoarchaeological research at European castles to target archaeologists, castle curators and heritage management policy-makers, so that our guidance can influence their future excavation strategies and conservation plans. Geoarchaeology applies environmental science techniques to archaeological research questions.  Soil micromorphology is a geoarchaeological technique that enables us to conduct a micro-excavation by analysing a slide, or ‘thin section’, produced from an intact, small block collected from the archaeological stratigraphy, so the inclusions are still in situ. The slide is analysed using a geological polarising microscope. Under the microscope, we record the same soil properties as archaeologists do in the field and additional ones, to understand the processes by which materials were deposited, how they have decayed due to chemical weathering, and how later human and faunal disturbances have affected the stratigraphy.

FIGURE 2. Micromorphology sampling of turf modifications to the curtain wall at Dunyvaig Castle, Islay, Scotland. Photo credit The Dunyvaig Project
FIGURE 3. Micromorphology sampling of occupation deposits sealed below later masonry and collapse during excavations ahead of conservation work on the eastern range of Cēsis Castle, Latvia. Photo credit Alex Brown
FIGURE 4. Micromorphology or ‘thin sections’ from  Elbląg (Poland) and Karksi (Estonia) castles. Photo credit Magnus Elandner

Soils and sediments are the backbone of the archaeological record. Our data revealed important stories within the soils about the early development of castle sites, the activities that took place inside different areas of the castle and how they may relate to the activities in a castle’s hinterland. We noticed that the preservation of the stratigraphy was exceptional owing to its burial under later masonry structures, masonry collapse and successive occupation layers; a phenomenon which is also observed on urban sites and Near Eastern tell sites.

Our results highlight the great potential and requirement for scientific analysis of these deposits when they are uncovered by excavation and/or building activities. These building activities include the conservation and renovation of the standing architecture and the removal of rubble, which can unseal buried archaeology affecting its preservation. Our analyses have revealed microscopic evidence for in situ animal husbandry and horse stabling, as well as the types of fodder that livestock were fed, crop processing activities, and periods of soil formation where areas of a castle have been abandoned or changed use. For example, at Castillo de Molina de Aragón, Spain, microstratigraphic data show the changing nature of the occupation the citadel, from the Islamic to Christian period which could possibly indicate a change in its role from a clean, well-maintained space to one where livestock roamed. The Islamic plaster floor surface was kept clean and very little domestic refuse accumulated  ̶  a soil formed over it, followed by a substantial destruction horizon. In comparison, units of discarded material and trampled floor surfaces characterise the (Re)Conquest phase, c. 1154, and a series of plaster floor surfaces with associated occupation residues containing herbivore dung showing that livestock were present, which probably relate to modifications during the thirteenth and fourteenth centuries.  

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Unravelling the past of Hoghton Tower

Steven Spencer and Elena Faraoni of the Hoghton Tower Preservation Trust, look at the results of their work, funded by the Castle Studies Trust in 2019, in trying to find out more about Hoghton Tower in Lancashire.

Hoghton Tower sits 650 ft above sea-level in the heart of the Lancashire countryside. The stories of its visitors and family members are documented and shared whether it be in books, portraits, family albums or documents in the Lancashire archives. But there is one story which has always intrigued us and that is: what was the first tower of Hoghton Tower and where was it? It is clear when looking at the building today that this, like many other historic houses, is a ‘patchwork’ of different projects by different generations interlaced and blended…but where did Hoghton Tower start? Where was the original tower?

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 There are some clues: the ageing of the stone, the position of the well house, family stories passed down the generations, the shape of the windows and a mysterious mound of stones on the north side of the buildings. One of these stones has an intriguing mason’s mark… Spurred on by the interest of a group of our amazing volunteers who had just finished some research into historic graffiti and masons’ marks it was time to do some investigation under the guidance of Dr Mike Nevell and his team at Salford University. We designed a research project based on archaeological digs, building recording, geophysics and archives research based on the key exam question “where was the great keep of the Hoghton Tower hill?”

North side of Hoghton Tower (copyright Hoghton Preservation Trust)

 Thanks to the grant from the Castle Studies Trust, work quickly got underway. Through a series of Salford-led workshops, the team surveyed, recorded and reviewed old photographs and pictures.

Then there was the wonderful five-day archaeological dig.

As they passed through the perimeter fencing on to the dig site, the usually mild-mannered volunteers underwent personality transformations as pairs of friends and even married couples were ‘pitted’ against one another. Was this the site of a 14th century Pele Tower, a 1643 victim of the First Civil War, who would make the crucial find?

Under the patient guidance and control of the Salford team, the test pits were marked out and the excavations began, to many this was the chance of a lifetime and was eagerly embraced. Each find was announced with enthusiastic shouting from the discoverer and muted derision from those yet to make a meaningful contribution.

Hoghton Tower Gatehouse from inside the courtyard (copyright Hoghton Tower Preservation Trust)

As the week progressed, 14th to 19th century finds were unearthed, thankfully shared out between the eight test pits. Clay pipe bowls (1640 to 1680), a musket ball, heat affected glass, sherds of medieval pottery and fragments of medieval roof tiles. Below a stone rubble layer, evidence of a stone-built structure was revealed in the form of large dressed stone blocks, together with walls and a stone flagged floor.

Spurred on by the whole experience, and encouraged by the de Hoghton family, the volunteers have produced and presented an ‘Outdoor History’ tour which aims to share the latest thoughts and discoveries. 

Was this the site of the Hoghton Tower? Did we find anything categorical? Well yes and no. The archaeology revealed previously unrecorded stone structures. These together with the artefactual evidence were able to confirm that this part of the hilltop was occupied during the late medieval/early post-medieval periods. The geophysics also gave us other areas that warrant more digging and researching. So, some confirmation but also a lot more to understand and discover on this windswept hill!

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Featured image of Hoghton Tower, copyright Hoghton Tower Preservation Trust

Geophysics and Castles: An Introduction

By Dr Kayt Armstrong, MCIfA

Hello! A personal introduction before we get down to the geophysics. I’m Kayt Armstrong, and I am an advisor to the Castle Studies Trust as a specialist in the use of geophysics in archaeology. I am also a member of the board of the International Society for Archaeological Prospection, and I represent the UK on a European research network about soils and geophysics in archaeology. I obtained my PhD in Archaeological Geophysics from Bournemouth University in 2010. I have worked in the UK and Europe (Greece and Italy) since that time, conducting archaeological geophysics in a variety of research and developer-led contexts. I help the CST evaluate funding applications that have geophysical elements, and also comment on the reports from any resulting work.

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What is ‘Geophysics’?

If you were a Time Team enthusiast, you probably already know the answer to this one!

Geophysics is the study of the physical properties of the earth (or other planets – you can do astrogeophysics!). It is an extensive term that encompasses whole planets, right down to understanding the microstructures of stone. Archaeological geophysics falls into ‘near-surface geophysics’, which studies the first 30m or so of the ground. In fact, commonly, archaeological geophysics is only really concerned with the top 2m or so; material in the topsoil, rather than the bedrock.

Geophysicists use a variety of methods and instruments to get information about the physical properties of the ground, such as its ability to conduct electricity, or its magnetic properties. Small variations in those properties can then be mapped. Buried archaeological material causes variations in the properties in predictable ways. This means we can map buried archaeology using these methods, without having to dig everything up.

When it comes to Castles, there are three main geophysical methods: earth resistance (‘resistivity’), magnetometry, and ground-penetrating radar (GPR). All three techniques look at slightly different aspects of the sub-surface, and all three have benefits and weaknesses. It is really best to combine methods to get as complete a picture as possible, as they will all tell you slightly different things.

Magnetometry

Magnetometry is probably the most commonly used technique in archaeology. It uses sensors to look at small variations in the strength of the earth’s magnetic field, to look for changes caused by buried material. The soil on sites where humans live becomes more magnetic over time, due to things like fires for cooking and warmth and the fermentation of waste material. This material becomes the fills of cut features like pits and ditches. These end up more magnetic than the soil they are cut into.

Structures used for processes involving heating, such as kilns, furnaces and ovens, become even more strongly magnetised and have a very characteristic appearance in the data. Similarly, fired ceramic building materials like brick or tile have a distinctive signal, as do igneous or metamorphic rocks (those modified by heating during their formation).

Magnetometry survey of Pembroke, funded by the CST in 2016

Magnetometry is very fast, covering upwards of 10ha a day if using the latest equipment. It is also relatively easy for community groups to employ. However, the pace will be somewhat slower using hand-carried single sensors. The plus-sides are the speed of survey and the wide variety of archaeological features that can be detected and mapped. The downsides are that this method is strongly disrupted by ferrous material in the survey environment, and has problems on igneous and metamorphic geologies as happened with the survey of Tibbers in 2014. It is also not very useful for mapping stone remains that are not strongly magnetic (such as some sandstones and most limestones). Modern infrastructure within or adjacent to the survey area has a far greater impact on the results than any buried archaeology (as happened in the Wressle survey of 2019), masking it from detection. It is practically not possible to use this method in urban areas. This method cannot detect smaller structures if they are buried more than about 2m below the ground surface. Features in the first 2m can usually be detected but the size of the anomalies that can be distinguished depends on the resolution of the readings taken. However, this method doesn’t let you understand the depths of the anomalies, and so isn’t as helpful on multi-period sites.

Earth Resistance

Earth resistance examines how easy it is to pass an electrical current through the ground. The resistivity of the subsurface varies in relation to several properties. Still, the most substantial effect is caused by variations in moisture content. The fills of cut features like pits and ditches (as witnessed in the 2018 survey at Laughton which showed a possible ditch, confirmed in the 2019 excavation and of Tibbers which lead to the discovery of a new inner bailey) tend to have a more open texture than natural soil. They usually also contain more organic matter. This means they are generally wetter than the ground they are dug into. Conversely, buried structures like walls and floors, are usually much drier than the material surrounding them, because they can’t absorb as much water.

Earth Resistence Survey at Tibbers, co-funded funded by the CST in 2016. Copyright HES

This technique can be applied in two ways. You can collect a grid of readings over a flat area to examine the first 2m or so, producing a plan view. You can also collect long lines of readings with increasingly wide measurement points. This is called ‘Electrical Resistance Tomography’ or ERT, and produces vertical pseudo-sections through the ground, and can reach greater depths, typically in archaeology 8-10m.

The plan-view method typically involves 2 probes on a mobile frame, and two remote probes connected by a cable. 2 of the probes inject a current, and two measure the resistance to it. It is especially useful for mapping buried stone structures. It is therefore handy on ‘Castle’ sites where multiple building phases can be expected. It is relatively slow, however, and relies on being able to insert probes into the ground to get the readings. This is fine on a lawn or field, and it can work on paths and gravel, but the results get very noisy, and it isn’t possible over flagstones or tarmac or concrete. You also need to be able to manoeuvre the cables and place the remote probes at an appropriate distance. This method also doesn’t let you understand the relative depths of various anomalies.

ERT is less commonly used in archaeology, but it has some specific applications in the study of large defended sites. Because it can resolve structures at a greater depth than the plan-mode, it can be used to examine the construction of large structures. This includes moats, earthen banks and buried fortification walls, and other such features. If multiple adjacent profiles are collected, the data can be combined into a 3D model of the subsurface, which can help resolve questions about the construction sequence of a site.

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Twin-probe (plan-view) resistivity survey is relatively straightforward to carry out. It doesn’t require as much skill on behalf of the instrument operator as magnetometry does. It is however, slow and laborious. The equipment is relatively cheap, and data processing and visualisation are relatively simple. This method is rarely used in the commercial sector these days but is an ideal research tool. Community groups have produced excellent research using this technique. The ERT method requires specialised equipment and a trained collector. The background knowledge needed to correctly process and interpret the data is also high.

Ground Penetrating Radar

GPR only made the odd appearance on Time Team, but in the last decade or so advances in computing (mostly increasing miniaturisation of components, and improvements in battery life) have led to a new generation of GPR kit that is more flexible and affordable.

GPR works a lot like sonar or the sort of radar employed by aircraft. A transmitting antenna sends out radio waves focused into the ground. These propagate downwards and are reflected by abrupt changes in the material of the subsurface. For example, when the waves leave a stone ceiling and move into the vault, some of the waves will be reflected back up. Some will continue on, to encounter the floor of the vault, and anything below it. The reflected waves are collected by a receiving antenna (usually in the same ‘box’ as the transmitter, a fixed distance apart). The strength of the returned waves, along with the time (in nanoseconds!) it takes for them to return is logged and plotted. This is a single trace.

Ground Penetrating Radar Survey at Shrewsbury Castle 2019, funded by the CST

The antenna is dragged along a line, and a series of traces are collected at a small interval (usually every 5cm or 2cm), which are combined together to make a profile. This is effectively a vertical slice through the ground. These are a bit difficult to read because the radio waves emit in a curved shape, so they actually travel in front of and behind the antenna, not just directly under it. This creates distinctive hyperbolas in the data. The depth of signal penetration and the size of the objects you can detect varies with antenna frequency. Depending on the frequency of the antenna, you can look very shallowly and resolve things that are a centimetre (or less) across. Very high-frequency antennas are used to assess the structure of concrete in civil engineering or to image different layers in mosaics and floor coverings. Lower frequency antennas cannot resolve smaller anomalies but can penetrate 10m+ to resolve much larger objects, such as former river beds, large walls or banks and ditches.

Groups of profiles collected in parallel lines can be combined together to make a 3D block of data. This can be processed in a way that allows different horizontal depth slices to be examined (as done at Fotheringhays; see time slice). A new generation of GPR system uses lots of antennas in an array to collect very high- resolution datasets (8cm in all directions), or arrays of different frequencies to quickly collect data with good resolution at multiple depths.

GPR requires a skilled operator to plan the work, collect the data and the process and analyse the results, but it is arguably the best technique for investigating Castle sites. This is because it tends to be good at detecting the sorts of things we would expect to be looking for, for example, voids, buried walls, culverts and surfaces. It can also be deployed inside standing buildings, to look underneath floors or behind walls. It can be used over tarmac and concrete (providing the concrete is not re-inforced!) and does well on most geologies, except for certain kinds of clays, and saline environments like estuaries.

The other advantage of GPR is that the data is relatively fast to collect, compared to earth resistance, and a broader range of features can be detected with it. It is also an inherently 3D method and allows complicated below-ground sequences to be visualised and interpreted. I have seen examples of staircases being visible in the data from 3D GPR, for instance. This technique has made the headlines recently, with the publication of a study of an entire Roman city, Falerii Novi, just north of Rome, by colleagues of mine from Ghent University and Cambridge University.

Geophysics and the Castle Studies Trust

Geophysical approaches form an increasing component of research proposals put to the trust, which is excellent to see! Geophysics can help to answer both broad and specific questions about castle-sites, without the potentially destructive process of excavation. Geophysics also has applications for the conservation of sites and planning for their future management. For example, in mapping the integrity of standing walls using GPR, or understanding the soils and material within earthworks to protect them from erosion in extreme weather events. They can help site managers decide whether an intervention is necessary, and can inform the design of any needed work. Geophysics can also play an essential role in the continuing life of these sites as homes or places of historical interest by mapping areas where conservation or building work is planned to ensure nothing is damaged by the work.

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Featured image is of the earth resistance survey at Pembroke in 2016