Of the precise date of the first Shell Keep erected in these islands we have no definite record; it is very doubtful if any saw the light during the reign of William the Conqueror or Rufus, although many examples could be found at that time upon the Continent. We know that certain Castles, such as Carisbrooke, Lincoln, and Totnes, had developed Shell Keeps prior to the termination of the reign of Stephen, and that Windsor, Berkeley, Arundel, and a number of others were furnished with the same not very long after, so that the age of the Shell Keep may roughly be ascribed to the twelfth century. One must not infer, however, that every example of a Shell Keep dates inexorably from that age, because, having proved its efficiency, it became a recognised method of defence, and Lewes and Durham were endowed with Shells as late as the reign of Edward III. The Shell Keep is always placed upon a mound, either natural, structural at the time of erection, or a pre-existing motte, but by far the greater number of mounds are artificial. The configuration of the earthwork suggested the shape of the Shell, being either circular, oval, or, as in the case of York and probably Warwick, that of a quatrefoil. The majority are polygonal, the sides not necessarily of equal length, and few of them exceeding the duodecagon in number. The diameter varied from 100 feet to 30, seldom more or less; the thickness of the wall was from 10 feet to 12 feet, and the foundations were carried from 4 feet to 6 feet into the soil. This wall was not built upon the extreme edge of the plateau, but generally a few feet from it and carried upwards to a height of between 20 feet and 30 feet, steps of wood or stone upon the interior face giving access to the rampart. Being essentially in one compact mass, without vertical breaks of any great extent, and homogeneous in construction, the Shell Keep was specially adapted to crown the summit of an artificial mound. The interior area was occupied by buildings, generally abutting upon the Keep walls; in early examples these were constructed of wood, but subsequently almost entirely of stone to lessen the danger of conflagration.