The ramparts upon the summit of a Rectangular Keep were carried upon the walls themselves, the latter, as a rule, being sufficiently thick for the purpose without corbelling outwards. The parapet was either continuous or embattled. A roof, at times covered with lead, was carried over the central opening, and the uppermost floors were invariably borne upon massive wooden joists. The lowest floor was generally free from timber, being constructed of masonry carried upon the arches of a crypt, but in those cases where the whole structure was borne upon a solid foundation of masonry spread upon the entire area of the site, this might be dispensed with. Some existing crypts are not coeval with the building, but were added at a later date, that at Richmond, for example, dates from the Decorated period. As a general rule the Keep contained a well which was sunk through the foundations and carried upwards in the central dividing wall to the various floors, but examples occur where it is placed in the enclosure. Most Keeps were furnished with an oratory or private chapel, one of the most famous being that in the Tower of London, while those at Newcastle-upon-Tyne, Colchester, and Guildford are well known. In the later type of Keep this feature is absent, the tendency being to erect all buildings used during times of peace within the enclosure.
The reduction of such a Keep as we have outlined was almost impossible in the Medieval age except by famine; the outer minor defences, however, were not proof against the missiles of the trebuchet, onager, and other petraries, and would invariably succumb. But with regard to the massive structure of the Keep, the largest stones could be hurled with but small results; and the few narrow openings in its walls presented but meagre opportunities for a successful admission of the falarica, quarrel, or arrow. To carry it by direct assault would be at all times a forlorn hope.
We thus see that the Rectangular Keep was essentially a structure for passive defence; and during the time that provisions lasted it was practically impregnable. Built upon the living rock, as they generally were, it was an impossibility to mine them; even if attempted, mine could be met with counter-mine, and the ram and sow might in vain essay to make any impression upon such solid masonry. At the same time the garrison was to a certain extent incapable of inflicting much damage upon the besiegers except in case of assault; the steep shingle roof afforded no place for a military engine, and but scanty facilities for storage of rocks, stones, beams, and other weighty missiles for dropping upon assailants. The narrow entrance into the Keep prevented an effective sortie, and, if attempted, was a source of danger in retreat. During the three months spent by King John, in 1215, before the Keep at Rochester, his military engines produced practically no result upon it, but an effective mine succeeded in bringing down the masonry of one of the lower angles, and eventually part of the tower itself.
The great advantages perceivable in a solid Keep were so apparent that the addition of this feature to many castles of the Motte and Bailey pattern was deemed advisable, but only in a few places did the Keep stand upon the mound; Nottingham is an exception, but in nearly all other examples they occupied new sites, the tremendous weight of the structure rendering it inadvisable to trust it in that position. The superiority of the Keep over the Motte and Bailey Castle was well exemplified in 1102, when Robert of Bellesme, Earl of Shrewsbury, broke into rebellion against King Henry I. He possessed a fortress of the Motte and Bailey type at Quatford on the Severn, but this "Devil of Bellesme," as he was termed, had no confidence in his father's fortress, and transferred the stones higher up the river where, in the short period of twelve months, he built the imposing Keep whose massive remains, although sadly shattered at the time of the Commonwealth, still excite our admiration. It is erected upon a rocky site, protected by ravines upon three sides, and overhanging the river Severn upon the fourth. When besieged by the King it withstood all the efforts of the formidable petraries brought to bear upon it, and appears to have been practically uninjured when, at the expiration of a month, a portion of the garrison became disaffected by reason of the threatening nature of the royal messages, and managed to secure its surrender.
ROCHESTER CASTLE, KENT.
When a Keep was added to a castle of the Motte and Bailey type there does not appear to have been any regular rule as to its position. At Guildford it was erected upon the motte (though a little way down the slope), and also at Nottingham, Pickering, and York; at Clun in Shropshire the Keep was built partly on the motte, occupying the eastern slope, the mound apparently bearing a defence of the Shell Keep pattern at the same time. Gloucester Castle has been entirely destroyed in order to make room for a modern prison, but from existing records we learn that the Keep was an addition, occupying the centre of the former bailey, while the building at Newcastle also stood distinct from the mound. The Keep at Oxford stands upon the enceinte at some distance from the Shell Keep, while at Rochester and Canterbury the new additions were erected outside the original castle.