Much has been said of Norman dungeons, oubliettes, and subterranean vaults, damp and wretched, appliances of Norman tyranny. So far as these keeps are concerned in the matter, they never contain underground chambers of any kind. The basement floor is usually at the ground level, or at most two feet or three feet below it. Where the keep is built on rising ground it may happen that a chamber, the door of which is at the ground level, may have one wall half buried beneath the soil, but there is nothing beyond this. Prisoners of the common sort were not shut up in the keep, space there was too valuable. The basement could scarce have been used as a prison where it contained the castle well, and the mural chambers usually are barred inside. The rooms under the vestibule, and some of the lower vaults at Dover, and the crypts of the Tower of London, and at Colchester, probably were used as prisons. In the upper gallery at Dover there is a very evident prison. At Carlisle, where the basement has recently been used as a prison, it probably was not one originally. Large as some of the keeps were, they were not calculated to be held against a long siege or a blockade, and all the spare room would then be needed for provisions and stores. The earlier keeps are very plain. The Tower has not even a moulding save in the chapel, and an exterior blocking over its main tier of windows. No doubt it has been much mutilated, but, had the ornaments been cut off, the courses of freestone that carried them would still be distinguishable from the ordinary rubble masonry. Some of the later keeps exhibit rather rich details, though usually marked by much simplicity, about the doors, windows, and fireplaces. Such is the case at Rochester, Hedingham, Dover and Newcastle, and specially at Castle Rising, one of the most highly ornamented of keeps. Bamburgh has a fine doorway early in the twelfth century; Ludlow and Guildford some arcades; Porchester some good windows. The exterior of Norwich is, or rather was, rudely panelled in tiers of arches. Goderich, otherwise very plain, has an exterior string of hatched or chevron work. In these keeps the arches are usually full-centred, but sometimes segmental, and where flat there is commonly above the lintel a relieving arch with a recessed tympanum, as at Chepstow. At Malling, though there are no mouldings, the first-floor window on one side is the centre of five deep plain full-centred niches in the exterior face, which cannot have been meant for use, and in another face, also outside, are five other niches, all unpierced. Occasionally false arches are turned in the walls, as though a door had been closed up, or the possibility of a new opening provided for. Such are seen at Dover, Norwich, and Guildford. They are thought, but scarcely on good grounds, to be intended to invite an attack where the wall is specially thick.