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 ROMANO-BRITISH DERBYSHIRE an ' ala ' mounted. One fort might, therefore, house no more than 500 infantry ; another might have to accommodate 1,000 men with all their horses and stablemen. The shapes of the forts also vary, though less widely. They are almost invariably rectangles. But some are square, or nearly so ; others are definitely oblong. It is possible, though it is not certain, that the square shape was preferred in the earlier Empire, and the oblong in the second and third centuries. But local questions, as of space, must at all times have had great weight. The ramparts of the forts were banks of earth or of regularly laid sods, or walls of stone. In this point different periods had apparently different preferences. In the first century earth or sods were much employed, and sometimes a skin or facing of stone was added to ensure strength and a steeper slope to the rampart. During the second century the advantages of stone became gradually recognized. By the opening of the third century stone was universally used, and older earthen forts were rebuilt to suit the modern fashion. In front of the ramparts were deep V-shaped ditches, often two in number, and usually separated from the ramparts by level intervening bermes. As a whole, these defences admitted of considerable elaboration, and provinces probably differed somewhat in the treatment of them. In North Britain, for example, we meet with numerous ditches, massive ramparts, and substantial ravelins. Such occur hardly anywhere else in the Roman Empire, and we may be tempted to think that even in Roman days the Highland charge was uniquely fierce and irre- sistible. The entrance to the interior of the forts was by four or very rarely by six gates, disposed symmetrically, one in each side. From each of the four gates a street ran straight towards the centre of the fort, and other smaller lanes and passages divided the vari- ous buildings of the in- terior. These streets and lanes were all parallel to one or other side of the ramparts, and the in- ternal arrangements of the fort preserved the same rectangular cha- racter as its outline. At the centre stood the chief buildings, the head- ORAIN ENTRANCE FIG. 5. HEADQUARTERS BUILDING AT GELLYGAER. (The part shaded was roofed.) 197