The earthworks at Castle Hill have been dated to a number of periods, and local folklore even links it to the Devil.
Castle Hill motte and bailey castle is one of the most impressive examples of this type of monument in the region; the motte is the highest in the county of Norfolk, and the surrounding earthworks, including those parts which have been levelled, are second only to those of Norwich Castle in extent.
The motte, banks, ditch fills and deposits within the bailey will contain archaeological information relating to the construction, use and abandonment of the medieval castle, and the limited excavations carried out on the site have also shown that evidence for the construction and intensive occupation of the Iron Age enclosure is also contained in the fills of the outer ditch, in the outer bank and in buried features in the interior of the enclosure.
Archaeological evidence relating to the prehistoric occupation is likely to be particularly well preserved in soils buried beneath the motte and banks, and it is probable that waterlogged deposits within buried features in the lower lying parts of the monument near the river will include well preserved organic materials and evidence for the local environment in the prehistoric and medieval periods.
The location of the monument, commanding the intersection of important land routes across the Thet and Little Ouse rivers, attests the strategic importance of the site during the later prehistoric and the later Saxon and early medieval periods, which led to the growth of Thetford as a major town between the 10th and 12th centuries. The presence of the friary, on the west side of the market of the later medieval town which developed on the north side of the river, gives it additional interest. The visually imposing earthworks, situated in a public park close to the centre of the town, are also an important educational and recreational amenity.
The monument, which is contained within two areas separated by a road (Castle Lane), is situated towards the eastern side of the town of Thetford, on a chalk rise on the north side of the River Thet. It includes a medieval motte and bailey castle incorporating remains of an earlier earthwork enclosure identified as an Iron Age fort. Also included is the site of an Augustinian friary which was established within the earthwork enclosure in the later 14th century, on land to the south of Castle Lane.
In the area to the north west of Castle Lane and north of Market Street, the remains of the Iron Age enclosure and medieval castle include substantial upstanding earthworks. To the south east of Castle Lane, where there are no visible earthworks, an infilled ditch and other features and archaeological deposits relating to the prehistoric and medieval occupation of the site are known to survive below the modern ground surface.
The motte and bailey castle is believed to have been constructed shortly after the Norman Conquest, either by Ralph Guader, Earl of East Anglia until his rebellion in 1076, or by Roger Bigod, his successor as Earl. It was sited in a position to control important crossings of the rivers Thet and Ouse, as well as to dominate the town of Thetford which, at the time of the Domesday survey in the late 11th century, was among the six largest and most populous towns in the country. The pipe roll (exchequer record) of AD 1172-1173 records the destruction of a castle in that year, although it is possible that this reference is to Red Castle, a medieval ringwork to the south of the river.
The motte is a large, circular mound of chalk, approximately 25m in height and 90m in diameter at the base. At the summit is a sub-rectangular platform about 25m in diameter surrounded by a bank of chalk rubble approximately 2m in height with an opening on the north west side. The platform would originally have supported a timber tower, evidence for which will survive below the ground surface, and it is likely that the bank represents the footing of a wall or timber palisade. The base of the motte is encircled by a ditch about 18m – 20m wide which remains open to a depth of between 5m and 6m, and enclosing this on the north side is a very large double bank and ditch.
Immediately to the east of the motte and its encircling ditch is an area of level, open ground bounded on the north side by a double bank and ditch which are contiguous with the earthworks to the north of the motte. A sketch plan of the earthworks drawn in the first half of the 18th century shows the inner bank extending east of Castle Lane and turning southwards to enclose a sub-rectangular bailey with estimated dimensions of about 105m WNW-ESE by up to 90m. This eastern part of the bank is no longer visible, having been levelled in 1772, when the remains of an associated wall are said to have been found. It would, however, have been accompanied by a continuation of the inner ditch which, although now infilled, will survive as a buried feature. The probable line of the ditch is marked in places by a slight scarp. The same plan shows another bank running south eastwards from the motte on the south side of the bailey, where there are now a series of quarry pits visible as irregular hollows in the ground surface. Entry to the bailey is provided by a causeway across the earthworks on the north east side of the motte. The surviving inner bank on the north side of the bailey stands to a height of about 6m above the prevailing ground surface and the inner ditch remains open to a depth of approximately 10m measured from the top of the bank. The adjacent outer bank has a maximum height of about 3m above ground level, and the outer ditch remains open to a depth of not more than 2.5m. East of Castle Lane the outer ditch has been infilled, but observations made in foundations dug for a new house in 1987 have confirmed that it survives as a buried feature, running south eastwards towards the river and enclosing the eastern side of an area which would have been protected on the south side by the river itself and its marshy flood plain.
Limited excavations undertaken for the Norfolk Research Committee in 1962 produced evidence that the outer ditch is primarily of Iron Age date and that the medieval castle was therefore constructed within a much earlier prehistoric enclosure, utilising parts of the existing earthworks. The excavation revealed that this ditch was originally up to 4m in depth with straight sides and a flat bottom about 5m wide. It also revealed a smaller inner ditch buried beneath the inner bank, and this feature is thought also to be of Iron Age date, although there is evidence that it may have been recut during the medieval period. Further limited excavations to the east of the motte and in the part of the larger enclosure to the south of Castle Lane and east of Nuns Bridges have demonstrated the survival here of extensive evidence for both medieval and Iron Age occupation, including buried features of Iron Age date, such as pits and remains of timber structures.
Immediately beyond the outer ditch, in the area of Castle Meadow to the north of the castle bailey and east of the entrance causeway, the excavations discovered three sub-rectangular, chalk rubble foundations identified as being of medieval date, and these features, which are known to extend approximately 16m north of the ditch into Castle Meadow. To the north of the monument and immediately adjacent to Castle Street are the remains of a substantial historic earthen bank.
The Augustinian friary, which was located to the south of Castle Lane and East of Nuns Bridges, in what is still known as Friars Close, was founded c.1387 by John of Gaunt, Duke of Lancaster, on land granted by Sir Thomas Morley and Simon Barbour, and was a small house of not more than six friars. In 1408 the friars obtained a license from the Crown to demolish a house standing between the friary and the street, in order to extend the church and cloister and build a hermitage. The friary was dissolved in 1538, when it was reported to be `so bare that there was no earthly thing but trash and baggage’, and the buildings were later demolished. A plan of the foundations, drawn in the early 18th century, shows a broad nave, possibly aisled, a narrower, rectangular east end which would have contained the choir and presbytery, and a cross passage or `walking place’ between the nave and choir which was a characteristic feature of friary churches. A later plan, reproduced on a map of Thetford dated 1837, shows foundations for a steeple over the walking place and the base of a stair which would have led up to it. Although nothing of the friary is now visible, remains of the foundations are believed to survive below the ground surface.