Thurnham Castle crowns the point of a steep spur of the North Downs, which commands the Maidstone-Sittingbourne road and the Pilgrim’s Way to Canterbury. It is a motte and bailey castle with gatehouse and curtain walls in flint and traces of an oval or polygonal shell keep. The bailey was divided into an inner and outer ward, the latter formed by a bank and ditch on the steep southern slopes of the hill.
The castle has recently been acquired by Kent County Council and has been included in the White Horse Millennium Wood and Country Park Project. Much of the site has been cleared of undergrowth and public access has been provided.
Four miles to the N.E. of Maidstone are the remains of a Norman castle, built on the site of a British camp, which occupied the highest point of a very steep spur of the chalk hills there ; the central knoll was scarped, and a line of defence was raised, in advance, lower down the hill, commanding the road which passed at that point. The manor was one of the many belonging to Bishop Odo, the Conqueror’s half brother, by Arlette de Croz, and after his fall it was granted to a Norman, Gilbert Maminot, whose descendants took their name from this place. Robert de Thurnham held it (temp. Henry II.) and founded Combwell Priory; it is probable that he built this castle. His two sons died s.p. in the reign of John. In the reign of Edward I. we find the place in the possession of Sir Roger de Northwood, who, dying 13 Edward I., left a son, John, married to his neighbour at Leeds Castle, Joan de Badlesmere, the sister of Bartholomew, Lord of Leeds (q.v.). John de Northwood died 14. Edward II., and their descendants resided here for many generations. The castle, however, seems to have been destroyed at an early date. It was entirely a ruin in Leland’s time (Henry VIII.).
The structure covered an area of a quarter of an acre only, so its ruins are inconsiderable. The walls, built of flint, remain in some places, and are lying about in large fragments. There is a mound here, which denotes its pre-Norman date, and on it is still “a trace of masonry which may be the remains of a shell keep.” (Clark.) There are the ruins of two parallel walls of the gatehouse, with those of a low outer wall, ending in the buttress of a tower, which has dis-appeared. The Norman castle was built on the W. side of the mound.
From the remains of Roman urns found here, the place may perhaps have been a look-out station of the Romans on the approach road to Rochester from the S.E. coast. (Castles Of England, Sir James D. Mackenzie, 1896)