The donjon at Houdan was built about AD 1130. The inside plan is a splayed corner square, while outside it is multacircular, with four corner turrets (projecting). The two stories of the donjon are of an unusual height . The walls are carried up on all sides, 16 feet above th gutter to mask the roof. The original doorway is in the North turret, 20 feet above ground level, and was reached by a drawbridge from the curtain wall which passed near the donjon at this point.
Reference:- Sidney Toy, "Castles - Their Construction and History," (Dover Books -1985)
Redrawn and Enhanced -- Marcus Audens