The cliff-face forms the entire northern boundary, making it effectively impregnable from that direction. The builders did not stop there, however. On the eastern, southern, and western sides, where the ridge offered no such natural protection, they cut the rock and earth away to create a double line of banks with a wide, flat-bottomed fosse between them. A fosse is simply a defensive ditch, and in this case the deliberate shaping of the ground into an inner bank, an outer bank, and the channel separating them represents a considerable engineering effort on a site that was already naturally well placed.

The detail of the construction is worth pausing on. Stone facing was added to the internal face of the outer bank and to the external face of the inner bank, concentrating hard material precisely where it would have been most exposed to attack or erosion. Both banks taper and fade as they approach the cliff edge to the south-east, which suggests the entrance may have been located at that point, a narrow approach where the ridge narrows and the drop becomes the deterrent. There is a gap in the inner bank on the southern side, but the outer bank shows no corresponding break, which raises questions about whether the two were ever intended to align, or whether the outer bank was added or modified at a different period. About forty metres to the south-west, at the base of the hill, a small U-shaped earthwork sits in the shadow of the main fort. Its interior measures roughly seven metres by five, and it is defined by a low earthen bank about two metres wide. Whether it was associated with the enclosure above, predates it, or served some entirely separate and now unrecoverable purpose, is not known.