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Motte and bailey castles are medieval fortifications introduced into Britain by the Normans. They comprised a large conical mound of earth or rubble, the motte, surmounted by a palisade and a stone or timber tower. In a majority of examples an embanked enclosure containing additional buildings, the bailey, adjoined the motte. Motte castles and motte-and-bailey castles acted as garrison forts during offensive military operations, as strongholds, and, in many cases, as aristocratic residences and as centres of local or royal administration. Built in towns, villages and open countryside, motte and bailey castles generally occupied strategic positions dominating their immediate locality and, as a result, are the most visually impressive monuments of the early post-Conquest period surviving in the modern landscape. Over 600 motte castles or motte-and-bailey castles are recorded nationally, with examples known from most regions. As one of a restricted range of recognised early post-Conquest monuments, they are particularly important for the study of Norman Britain and the development of the feudal system. Although many were occupied for only a short period of time, motte castles continued to be built and occupied from the 11th to the 13th centuries, after which they were superseded by other types of castle.

Yielden Castle is one of the most complete examples of its class in Bedfordshire. The small scale excavations of the late 19th century demonstrated the survival of the buried foundations of buildings and walls. These are also indicated in many areas by platforms and other minor earthworks. The buried deposits within the castle (particularly within surrounding ditches) will contain both artefactual and enviromental evidence related to the period of occupation and the landscape in which the monument was set. The motte and ramparts will also overlie earlier ground surfaces, preserving any evidence of former habitation or land use. The scale of the castle's defences illustrates the political and strategic importance of the site in the aftermath of the Norman Conquest whilst the later features in and around the castle illustrate its changing role in subsequent centuries. Fishponds, such as those at Yielden Castle, were a common feature of many types of medieval settlement and institution and were designed for cultivating and storing fish in order to provide a constant and sustainable supply of food. Those at Yielden are very well preserved and, together with the survival of the dovecote, reflect changes in the castle's purpose and priorities in the later medieval and post-medieval periods. These changes will also be documented in the changes to the surrounding enclosures and the wider parochial field system. At Yielden the adjacent agricultural enclosures are particularly well preserved and illustrate the close interdependence between major defensive and administrative sites and the broader agricultural economy.

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The monument is located in the valley of the River Til, on the eastern side of the village of Yielden, and includes the remains of a medieval castle mound with baileys to the north and west, and a series of banked and ditched enclosures arranged along its eastern side. The mound, or motte, which provided the main stronghold within the castle, lies some 100m east of Yielden High Street. It is steep-sided and roughly oval in plan, measuring approximately 55m by 70m around the base and 18m by 40m at the top. The mound rises in two stages from the south, the first stage forming a D-shaped platform about 5.5m above the level of the western bailey. Access to this stage was provided by an earthen ramp, which remains visible as a slight break in slope on the western side of the mound. To the north, the motte rises by c.2m, approached from the lower stage by a broad hollow in the intervening scarp. The summit is flat and about c.10m square, surrounded by a low bank. This area was partly excavated in 1882 revealing the robbed-out foundations of a substantial stone keep or tower. The eastern side of the motte descends into a broad ditch, or moat, which continues around the bailey on the western side of the mound, indicating a single period of construction for both these features. The ditch varies between 15m and 25m in width, averages 1.5m deep, and is seasonally waterlogged. It encloses a roughly rectangular area, measuring 90m north to south and 45m east to west, with the motte protruding from the north eastern corner. The western half of the bailey contains numerous low earthworks, mostly sub-rectangular, and considered to be building platforms. The bailey was originally enclosed by a continuous bank, which now survives to a height of 1.5m-2m along the southern and south eastern arms, and as a slight earthwork elsewhere. The northern section of this rampart, which during the period of occupation is thought to have supported a timber palisade, continues about half way up the western side of the motte. Exploratory trenches dug within the bailey in 1881 exposed stone debris from demolished structures and uncovered foundations for a retaining wall on the inside of the rampart. Associated with this wall, in the south west corner of the bailey, lay the base of a large circular structure, which was considered to be a bastion, or fortified tower. The second bailey lies to the north west of the motte, separated by the northern arm of the moat, and is thought to belong to a slightly later phase in the development of the castle. The bailey area is roughly triangular, with maximum dimensions of c.110m east to west and 70m north to south; its layout dictating the acute angle between the High Street and Melchbourne Road. A further high bank or rampart, measuring up to 14m across and 2m high, skirts the north eastern and western sides of this bailey. This is accompanied on the north eastern side by a large ditch, 10m across, which descends to c.2m in places. The eastern end of the ditch joins the earlier moat near the motte, although the moat is approximately 1m deeper than the bailey ditch at this point, and the latter is thought to have been dry. The interior of the bailey slopes downwards from the east, where several platforms indicate the locations of further buildings. In the centre are two slight depressions, each approximately 15m across. The southernmost of these is separated from the edge of the bailey by a broad hollow way leading toward the western arm and the possible location of a former bridge. A third depression, also dry, is located in the northern corner of the enclosure. This feature measures 25m by 8m and 0.8m deep, and is thought to represent a small fishpond. The western ditch is approximately 12m wide and 1m deep, and is thought to have been formed from the original course of the River Til, continuing to the south along the side of the earlier bailey and providing water to the rest of the moat. This ditch has been considerably altered in both medieval and modern times. The earlier changes, contemporary with the occupation of the site, include a widening of the southern part to form a large fishpond, adjacent to the western bailey. The enlarged section measures between 30m and 40m across and suggests a period of the castle's use when defence was no longer the primary concern. The pond is about 2m in depth, and largely dry, although containing deep silts deposited by slow moving water. A bank across the southern end, measuring 0.8m high and 40m in length served to dam the pond; and traces of the original outflow channel continue to the south as far as the boundary of Middle Farm. The junction of the outflow and dam was, however, truncated by the excavation of a small pond before 1881. To the west, the fishpond was contained by a bank, c.1m high and 10m wide, which extends for approximately 120m parallel to the High Street. A small mound or island located near the dam at the southern end of the pond was investigated in 1882, and found to retain the stone foundations of a medieval dovecote. A tithe map of 1842 shows the River Til to have been diverted away from the western ditch and pond, and lying between the eastern bank and the edge of the High Street. However, this channel was infilled around 1950, and a new course dug along the former route within the base of the fishpond and the west side of the northern bailey ditch. The development of the site from early military to later manorial use is illustrated by an arrangement of six rectangular paddocks aligned along the eastern side of the castle. These are defined primarily by ditches, varying between 5m and 8m in width and between 0.3m and 0.9m in depth, some accompanied by low banks formed from the upcast. The enclosures share a common eastern ditch which is orientated north east to south west, and located some 200m to the east of the High Street. The drainage channels separating the individual enclosures extend to the north west of this feature, and either abut or join the eastern side of the bailey ditches. The two most northerly enclosures are each about 35m square, whereas the third enclosure extends 20m further to the west following the curvature of the moat around the south east side of the the motte. The middle enclosure of these three is divided into two levels by a shallow scarp across the centre. Access to the lower, southern part is provided by a causeway across the main ditch to the east; and a second causeway spans the southern boundary ditch of the enclosure, near the moat. The fourth enclosure measures between 25m and 30m in width, and 70m in length; and abuts the eastern side of the moat surrounding the western bailey. The fifth, extends for 130m along the south side of the moat and terminates in a scarp at the edge of the fishpond. The sixth, and most southerly, enclosure continues for about 100m from the main, eastern ditch, and retains traces of ridged cultivation. The pasture to the south and east of the enclosures contains a wider pattern of ridge and furrow ploughing, much of which was reduced by ploughing during, and shortly after World War II. A sample of these earthworks, between 10m and 20m in width, is included in the scheduling to provide protection for their archaeological relationship with the enclosures and thus with the castle. At the time of the Domesday Survey, Yielden formed one of seven parishes on the Northamptonshire/Bedfordshire border held by Geoffrey de Trailly under the authority of the Bishop of Coutances. The parish later became the centre of the barony of Trailly, and the castle is thought to have served as its focus. It remained in the possession of the Trailly family through the 12th and 13th centuries, during which time Thorney Abbey was endowed with the parish church. However, in 1360 it was recorded as having "fallen intirely to decay". The following items are excluded from the scheduling: the surface of the public road at the north end of the site, together with the modern hardcore, culvert and drains below; the modern bridges spanning the channel of the River Til, and all fences and fence posts, although the ground beneath these features is included.

MAP EXTRACT The site of the monument is shown on the attached map extract.