The first characteristic of a concentric castle is the arrangement of its lines of defence, one within the other, two or even three deep, with towers at the angles and along the walls, so planned that no part is left entirely to its own defences. A wall cannot be advantageously defended unless so arranged that the exterior base of one part can be seen and commanded from the summit of another. A Norman keep or an Early English round tower could only be defended by the projection of missiles from the battlements, exposing those who discharged almost as much as those who received them. The employment of mural towers not only added to the passive strength of the wall whence they projected, but when placed within a bowshot distance enabled the defenders, themselves protected, to enfilade the intermediate curtain. By this means the curtain wall, that part of the work least able to withstand the strokes of the ram, became that in defence of which most projectiles could be brought to bear, whilst the towers, which from their distance apart were but imperfectly flanked, were from their form and solidity in little danger of being breached. By this means also material was economised. The walls were less thick, and the defence generally was carried on with more skill and less dependence upon passive strength. In this indeed there was nothing new, but not the less the principle was ignored in the construction both of keep-towers and their successors, the round towers, donjons, or juliets. But in the concentric system a good deal more than this was effected. The parts of the lines of defence were so arranged that the garrison could sally from one part, and so harass the attack upon another. Moreover each part, tower or gatehouse, and sometimes each stage of a building, was so contrived that it could be held separately for a short time. Also, from the concentric arrangement of the lines, a breach of the outer wall did not involve the loss of the place. The second ward, as at Beaumaris, Harlech, Caerphilly, or the Tower, was so narrow that in the event of its being entered from a breach, the assailants were exposed to an attack on either flank, in such a manner that their greater number could not be brought to bear, nor was there room to work a catapult or set up a malvoisin. In these castles, the keep, the main feature in a Norman or an Early English fortress, was dispensed with; it was developed, to speak anatomically, into an open court, strengthened at its sides and angles by gatehouses and mural towers, and having the hall and domestic apartments ranged against the wall along one or two sides. Around this inner ward was disposed a second ward, of narrow breadth, and broken up by cross walls. Sometimes around this second ward was disposed a third or outer ward, usually of large area, so as to accommodate the greater part of the garrison, the horses, and the neighbouring peasants with their cattle. In many