On the Duke of Gloucester's estate at Barnwell can be seen three successive manorial centers in close proximity. First there are the earthworks of a Norman motte And bailey, now hidden in a clump of trees. Then comes the massive stone ruin of Barnwell Vastle, built by Berengar le Moine about 1265-66. It seems that Berengar took advantage of Henry III's preoccupation with his barons to build a strong adulterine castle. Berengar later sold his new castle to Ramsey Abbey. It is said he was compelled to do so by Edward I as a punishment for building it without a license. Barnwell remained with the abbey until the Dissolution, when Sir Edward Montague purchased it. He erected the present house, Barnwell Manor, nearby.
The castle is an interesting example of thirteenth century military architecture with some delightfully experimental touches. On a smaller scale, it anticipates the great castles that Edward I would build in Wales in the following decades, and though it pre-dates Edward's coronation by several years, it is a rare English example of a pure Edwardian castle.
An unusually thick curtain, well preserved except for the loss of its parapet and a single breach on the west, surrounds an oblong courtyard. Circular towers project boldly at three angles, the fourth being occupied by a gatehouse. The two northern towers are quite eccentric as they both have a smaller round tower projecting from them, resulting in a figure-of-eight plan. The prime function of these subsidiary towers was domestic rather than military. They contained latrines serving the apartments in the main body of the towers.
The southwest tower has no projections, but its upper floors are square internally for greater domestic convenience. The latrine for this tower was accommodated in a more conventional manner within the thickness of the curtain.