It is probably late in the period. These keeps vary in dimensions from 25 feet to 80 feet and even 100 feet in the side, and they are usually from one and a half to two diameters in height to the base of the parapet. Many, perhaps most, have been raised a stage, as Porchester, Bridgenorth, Richmond, Brough, Brougham, Kenilworth, and Norham. In most of these cases the addition was made within the Norman period, and possibly contemporaneous with the substitution of a covering of lead for one of shingles or stone tiles. Usually the flat exterior faces are relieved by broad pilaster strips of slight projection from 5 feet to 10 feet wide, by 6 inches in depth, one at the end of and flanking each face, and in the larger keeps one, or even two, between them. The flanking pilasters are commonly placed so as to cover the angle, the two meeting; sometimes, however, they do not quite meet, and the solid angle is replaced by a hollow nook, occasionally, in late keeps, as at Castle Rising and Scarborough, occupied by a bold bead or engaged shaft. In all but the very small keeps these flanking pilasters are continued 8 feet to 10 feet clear above the parapet as the outer faces of square turrets, now usually ruined. At Hedingham one, partly perfect, remains. Those on the White Tower and at Rochester are in part original, and certainly represent, very closely, original structures. At Bowes there seems to have been but one turret, covering the stairhead. The intermediate pilasters usually stop either just below the base of the parapet or below an upper window. At Dover they are carried into the parapet and support slight internal recesses there, but this is very rare. The pilasters usually are divided by strings into stages, marking the levels of the different floors, and all rise from a common plinth, sometimes slight, but sometimes, as at Malling, Kenilworth, Guildford, Scarborough, and Norham, where one side rises from lower ground, the plinth is on that side 8 feet to 10 feet high, and has at Kenilworth and Norham a very bold base. The set-off and string-course are sometimes carried along both wall and pilasters. At Colchester and at the White Tower a projecting half-round forms the apse of the chapel, and the pilasters appear upon it; at Rochester a rounded corner is solid, save at one story, but this is probably due to a reconstruction. The pilasters are one of the most marked features of the Norman style, and their presence at once distinguishes a keep of that period from the fine fourteenth-century towers, as Borthwick and Lochleven, found in Scotland, as well as from towers of Early English date, where the pilasters are bolder and narrower, and often, as at Exeter Castle, chamfered at the angles.