From the National Heritage List for England: Details The monument includes a motte and bailey castle and is situated on a spur of the North Downs above the Pilgrim's Way historic routeway. The motte takes the form of a generally steeply-sided conical mound 70m in diameter at the base with a flattened top 22m across. In a ring around the top of the motte are traces of a shell keep, with one 2.5m section of flint walling more prominent on the south-west side. On the western and northern sides the motte drops some 5m to a ditch, now largely infilled by eroded soil from the mound, which provided additional defence for the keep on the mound. This ditch, most clearly visible for 150m to the NW of the motte, measures between 5m and 9m in width and now reaches no more than 1m in depth. On the eastern and southern sides the land slopes less steeply and the foot of the motte is less clearly defined. The bailey area to the west of the motte is defined by a thick flint wall, much of which has been reduced to footings by robbing of the stone but which survives to an impressive 3.5m in height along the northern edge. Integral to this northern curtain wall, and beside the edge of the motte ditch, are the remains of a gatehouse 10m long by 5.5m wide with blocked Norman-style archways. The overall size of the bailey, as defined by the curtain wall, is 55m N-S by 35m E-W. Beyond the curtain wall the land drops sharply to the road on the western side. To the south, a quarry of uncertain date has undermined the boundary wall. The quarry is likely to have had its origins in providing the flint nodules for the building of the castle but is excluded from the scheduling. Reasons for Designation 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. Thurnham Castle survives well and exhibits a wide diversity of features such as the gatehouse and and stone built shell keep remains. It retains considerable potential for the recovery of evidence of the nature and duration of the use of the castle.