In most keeps there is a well-stair in one angle, which commences at the ground-level, supplies every floor, and terminates on the roof under the turret-head, but has no communication below with the exterior. This was so originally at the White Tower. There are also other staircases at other levels, beginning on the first, or even the second, floor, and not always reaching the roof. As a rule, one stair, descending to the storeroom, seems to have been enough; but it was thought an advantage to have two or three ascending from the hall or upper floor to the ramparts, for readiness during a siege. At Canterbury there are also two well-staircases in the side walls; and there is one in London. These well-staircases are from 8 feet to 12 feet in diameter, and lighted with loops. Usually they communicate with each floor through a sort of lobby. The stairs are always of stone. As the steps at each angle do not suit the same level of the mural gallery, these latter are coaxed and accommodated, usually very clumsily. Now and then the arrangement is different, the stair stops at the first floor, and is continued at the opposite diagonal. Sometimes, as at Carlisle, Ludlow, and Bamburgh, where the outer door is at the ground-level, the staircase commences in the side of the doorway, and ascends straight in the wall, and, on reaching the next angle proceeds as a well-stair. This is so at Chepstow, where the upper ascent is probably an addition. At Carlisle and Ludlow the staircase stops at the first floor, and is continued at another angle. At Brough, where the outer door is on the first floor, a narrow straight stair rises in the wall to the second. At Prudhoe, it is continued round two sides, and at Richmond. The direction of the staircases may be always detected by the position of the exterior loops, and the general rule seems to be to limit the approach to the stores and main floor, and from the latter to give a free access to the ramparts. The curves and angles in those narrow staircases facilitated the defence of them.