 After 1161, important reinforcement work saw this keep raised and augmented; the wooden palisade of the motte converted to stone, thus forming a chemise; and the outer wall of the bailey was completed in stone with flanking towers.The octagonal keep is considered one of the best preserved examples of a shell keep.A second keep, cylindrical in shape, called the Prisoner's Tower ("tour du prisonnier"), was added to the outer wall of the castle at the start of the 13th century, following the French conquest of Normandy.