There were also many true stone castles, massive, heavy, rectangular keeps, erected by the Normans. These castles (see Rochester Castle), which depended at first for their impregnability on the thickness of their walls, not on any series of fortifications or ingeniously constructed enceinte, were the type favoured by the Normans. They built the " shell " form from compulsion rather than from choice, as is proved by the fact that, while the rectangular form of keep is found sometimes on an old, sometimes on a new site, there is no single instance of the adoption of the " shell " where the castle was erected on entirely new ground. The massive heavy tower could be safely founded only upon solid ground, whereas the lighter and more evenly distributed weight of the " shell " could be carried by the less solid ground of the artificial or semi-artificial mounds of the " motte and bailey " which they were designed to strengthen.
The small peel towers—square towers of masonry with three floors, used as a dwelling and a refuge for raiders, the remains of which stud the Scottish border—are mostly small editions of the great fortresses of Colchester and London.
The keeps of the Tower of London, Colchester and perhaps a few others date from the eleventh century.
In the Conqueror's day the vast quadrilateral Tower of London stood by its own strength : any other defences which existed must have been very unimportant. The walls are fifteen feet in thickness at the base while, high above the ground, they are fully eleven feet thick. The height of the building, which consists of four stages, is ninety feet. The tower is divided internally into two parts by a longitudinal wall, east of the centre, ten feet wide. There are two mural passages, one in the first and one in the second storey. The wall of the third storey is pierced all round with a gallery, having a barrel vault in the thickness of the wall. There is a well-stair, or vice, in a round turret at the north-east corner. This was the chief the floors, but vices were also means of communication between all made from the second floor to the roof in the square turrets of the north-west and south-west angles. The gloomy basement served as a storehouse for the large quantities of food which must have been required for a long siege : the first floor, or entrance floor, which is hardly less gloomy, must have been intended for inhabitation, perhaps as a guard room for the garrison, as it is fitted with chimney-flues. The second floor contains the large chapel of St. John and the banqueting chamber ; the third, or " state " floor, comprises the council-room and the king's apartment. There are, of course, other small rooms in each stage.
The Keep, Tower of London
The main well-stair in the north-east corner of the smaller vices:the Chapel of St. John, and the dividing wall.
The experiences of the First Crusaders brought warriors of the West face to face with fortifications superior to the Norman keep, which had certain weaknesses. First, it was impossible for the besieged to fire except at right-angles with the wall ; secondly, the salient angles of the masonry were liable to destruction by sap and mine. These reasons alone probably account for the appearance of circular or many-sided polygonal keeps or donjons. These were at first essentially the same as square keeps, differing only in form. The circular keeps were often known as juliets. Two good examples can be studied at Conisborough, built during the last quarter of the twelfth century (Henry II.'s reign), and at Pembroke, commenced in the year 1200. This type, however, was regarded for some time as something of a foreign fashion. We have no early work in this style by English masons that can compare in grandeur with the impregnable towers of Coucy in France.
built in the reign of Henry II. A regular cylinder to which are applied six bold buttresses which narrow slightly outwards and rise above the parapet in turrets.
Plan of Conisborough
Norman Keep, Pembroke. Built about 1200