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. [Illustration: ROCHESTER CASTLE, KENT.]