fort with stone walls necessarily a late construction. Other considerations must be taken account of—conditions of permanency, the presence close at hand of the necessary materials, the predilections of individual commanders, and the like. The fort at Gellygaer, for instance, with its rampart faced on both sides with masonry, was probably erected in the first or very early in the second century, while the earthen ramparts of Birrens with equal probability date from a period at least some fifty years later. In the general development of defensive works, no doubt the earthen fort preceded the fort defended by stone walls, but we know for certain that the transition did not take place everywhere simultaneously.
The Later Fort
The later fort at Newstead, inclusive of its defences, covered an area of 20.824 acres. The interior space was 15.716 acres. It is thus much the largest Roman fort yet excavated in Scotland. Birrens measures only about four acres, Camelon and Lyne each nearly six acres, Castle Cary three and a half acres, Rough Castle about one and a half acres, and the later fort at Bar Hill just over three acres. Inchtuthil and the great camp at Newstead have of course a much larger area. But they fall into quite a different category, having been constructed, as we saw above, for another purpose altogether. In Germany the same two main classes are to be noted. Besides the great legionary fortresses like Novaesium and Castra Bonnensia there were many small forts corresponding in their general dimensions to Birrens or Camelon, and holding perhaps a cohort of auxiliary troops as a garrison. Newstead, like a small minority of the German castella, is peculiar as coming, in respect of size, between the cohort camps and those intended to accommodate a legion. There is therefore no doubt that it must have been of greater importance in the scheme of advance than the ordinary frontier-post, whose object was merely to guard a ford, or to protect the line of communications.
The Ditches
The fort itself was of the usual rectangular shape, and greater in length than in breadth, measuring from the inner face of the wall 810 feet in the one direction by 720 feet in the other. The corners were rounded, and the four gates, one in each side, were placed opposite each other. The defences consisted of three parallel lines of ditches, a wall, and a rampart. These had to be planned entirely from sections cut across them, as no trace of their outline remained above the surface. On the north, observations were seriously interfered with by the high-road, which covers the greater part of the line of the fortification. At the north-west corner, however, the
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