The cliff did half the work: the northern edge of this roughly circular enclosure, measuring about 23.5 metres north to south and 23 metres east to west, is defined not by any human-made barrier but by the cliff-face itself, which drops sharply down to a river valley below. It is a practical, almost economical piece of thinking. Why dig a ditch on a side that nature has already made impassable?

On the three remaining sides, east, south, and west, the builders cut away the ridge to construct a more conventional set of defences. A fosse, which is a ditch dug as part of a fortification, runs between an inner and an outer bank, with the fosse measuring up to four metres wide and over two metres deep in places. The outer bank still stands to around two metres on its external face, while the inner bank, though lower on its interior, rises nearly four metres on the outside, making the full drop from bank top to fosse floor quite considerable. Both banks were reinforced with stone facing, on the internal face of the outer bank and the external face of the inner, suggesting the builders were not content to rely on raw earth alone. There is a gap in the inner bank on the southern side, but no corresponding break in the outer bank, which complicates the question of where the entrance actually was. A more likely candidate is the south-eastern corner, where both banks trail off towards the cliff edge, leaving a narrow corridor between earthwork and drop. About forty metres to the south-west, at the base of the hill, a small U-shaped earthwork survives; its relationship to the fort above it, and whether it belongs to the same period at all, remains an open question.