The first building work is dated to about 1095, and consisted of a motte, which was enclosed in a spacious courtyard or bailey. Henry I of England, Duke of Normandy, added an octagonal stone keep to the motte. 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.