An unaltered Norman castle is very rare, if indeed such exists at all. It is, however, certain that the keep had an _enceinte_ defence and ditch, the latter sometimes part of an earlier earthwork; and in the base court thus formed were stabling and barracks, and other subordinate accommodations. These buildings were at first often of timber, and the _enceinte_ a stout palisade, the object having been to afford protection to the garrison as rapidly as possible. Both at Dover and Windsor the _enceinte_ wall, part of which is of late Norman date, stands upon the scarp of the ditch of an earlier earthwork, the solid chalk of which, as at Arques, is traversed by subterranean galleries. Where, as at Cardiff and the Tower, the wall is of great strength, and of the twelfth century, it is probable that the palisade was retained longer than usual, and the wall now seen the first constructed. No regular Norman wall would so soon have required reconstruction. Where the Norman wall was of light construction or insufficient area, it had to be removed, and in the larger works replaced by a double and concentric ring of defences. These additions, usually due to the reigns of Henry III. and Edward I., show that military engineering had made great progress, and that less dependence was placed upon passive strength, and more upon the skilful distribution of material. Having regard to the state of the military art at that period, and to the cross and long bows, catapults, rams, scorpions, and movable turrets that formed the weapons of attack, it would be difficult to improve upon these concentric works, either in general design or in detail of construction, or to show greater skill in flanking defences than appears at Corfe, Caerphilly, Conway, or Beaumaris, or in other of the castles built in the reigns of Henry and his son. This science, so successfully grafted upon the pure Norman works, was no doubt in some considerable measure derived from the East, where Cœur de Lion seems to have acquired the skill displayed in the construction of Château-Gaillard, and which, in the opinion of M. Le Duc, places him at the head of the military engineers of his day. When, having crossed the Thames, the Conqueror marched in person to complete the investment of London, he found that ancient city resting upon the left bank of its river, protected on its landward side by a strong wall, a Roman work, with mural towers and an exterior ditch.