Where human labour ends, the cliff takes over. From the north-east around to the south, a sheer rock face replaces any need for an earthwork, and only along the remaining arc did the fort's builders cut a fosse, a defensive ditch, nearly four metres deep, backed by a low counterscarp bank on the outer side. The combination is economical and deliberate: do the minimum where nature already does the work.

The enclosure is roughly oval, measuring just over thirty metres east to west and twenty-three metres north to south, which places it at the smaller end of promontory-type forts, a category in which a natural feature, here the cliff and the river bend below it, forms one side of the defensive perimeter. What makes this example quietly unusual is the interior itself. The ground inside is not level; the western half sits approximately two and a half metres higher than the eastern half, giving the enclosed space a split-level character that is not especially common in earthwork enclosures of this type. Whether that difference reflects the natural topography of the spur or some deliberate shaping of the ground is not recorded, but it would have made the interior a more complex space than the modest external dimensions might suggest.