By degrees, as the Norman towers and shell-keeps fell out of fashion, they were succeeded by towers of a cylindrical form, known as Donjons or Juliets, and this change corresponds to the middle period of the Early English style in ecclesiastical architecture. Scientifically, in a military point of view, this was scarcely an advance, for the defenders of an isolated round tower could not concentrate their fire, and could only protect the foot of the wall by exposing themselves at its summit. On the other hand, with equal material, the round tower was stronger, and more difficult to breach or to bring down by a mine. Also, it admitted of being vaulted in every story, and was thus more solid and less exposed than the rectangular keep to being set on fire, either from within or by balls projected upon its conical roof. Usually, however, in England, only the basement was vaulted, as at Brunless. At Pembroke, the great round tower is vaulted at its summit, as are some of the cylindrical towers in the _enceinte_; but it is possible that these elevated vaults may be additions. At Coningsborough the basement alone is vaulted, and the vault is entered by a ladder from an aperture in the centre of the dome, under which is the well; the basement, as in Norman and Early English towers generally, being used as a store-chamber, and seldom, if ever, as a prison. In fact, in a single tower, whether rectangular or cylindrical, intended by its passive strength to defy attacks and to wear out the patience of a blockading force, an ample store of provisions was of the first consequence, and to their storage all the spare space was necessarily devoted. In those days, when the keep was the citadel, and not unfrequently used as such, prisoners were not kept within its walls. Dungeons there were none, save in a very few exceptional cases, and the basement or ground-floor was invariably occupied as a magazine. These donjons were usually entered at the first-floor level, either by an exterior stone stair or by one of timber; or sometimes, as at Pembroke, by a drawbridge, which dropped upon a detached pier, whence an inclined plane or a flight of steps descended to the ground level. In their elevated entrance, and in some other respects, these donjons recall the isolated towers erected during the decline of the Roman Empire, of which a good example, but square, remains at Autun. There are only two instances in England of the application of a fore-building to cover the entrance of these towers, but these—Chilham near Canterbury, and Orford in Suffolk—are late Norman, not Early English. Dolbadarn, in North Wales, a rather later tower, but cylindrical, has an exterior stair of stone, which, however, may be an addition.