Great towers looked imposing, but after a time they proved to be not very practical either for living or for fighting. During a siege, comfort and convenience were irrelevant, but during peacetime, living in the tower could be unduly complicated. Stacked rooms in the towers made movement through the building difficult, and the narrow spiral staircases in the corner turrets weakened the masonry at its most vulnerable spot. Furthermore, with the defense concentrated in a single tower, no matter how strongly built, the garrison had little flexibility during the siege. The rectangular building had a further disadvantage: a straight wall was difficult to defend because of the blind spots, especially at the base of the wall. Defenders had to risk their lives when they leaned out over the wall top to see what was happening. The crenels on the wall-walk were not sufficient, and wooden galleries (hoardings) had to be built out at the top of the wall in order to defend the wall (wall-walks and hoardings can be seen in Figures 12 and 22). Windows were needed for light and air, but they weakened the wall and so were often reduced to slits on the exterior. These arrow slits did not permit the archers to see the ground. By the end of the twelfth century, the builders of castles at Pembroke and Kenilworth perfected arrow slits by making sloping embrasures that permitted archers to shoot down at attackers on the ground. A very wide moat, such as the artificial lake and swamp (the “Great Mere”) created at Kenilworth, or a natural water barrier, such as the river Thames in London, effectively defended walls and countered attempts at mining. A good moat or ditch had to be too wide and deep to jump, wade, fill, or bridge. To reach the entrance, drawbridges were used, although the size of available timbers limited the size of the gap that could be bridged. Since doors were vulnerable places, elaborate defenses concentrated there. A sliding portcullis was an effective deterrent, but it required space for a counterpoise or windlass. Gatehouses solved this problem. Finally, a small semisecret back door or postern was usually built in the bailey walls. Also known as a “sally port,” the gate permitted the defenders to exit, engage in a brief battle, and return to the safety of their castle.