Where most early medieval enclosures rely on earthworks all the way round, this one uses the natural drop to the west as a fourth side, a practical economy that also tells you something about how its builders read the landscape. The western cliff face has, over time, exposed something else entirely: a souterrain, an underground stone-lined passage of the kind commonly associated with early medieval settlement, running beneath the inner bank and emerging in section on the cliff face as though the earth had been cut away to reveal a diagram.

The fort's shape has shifted, at least on paper, across successive Ordnance Survey maps. The 1842 six-inch survey recorded it as a hachured subcircular enclosure, while both the 1904 and 1940 editions show it as distinctly D-shaped, the straight side running roughly thirty metres north to south, with the enclosure projecting about eighteen metres eastward. Two earthen banks enclose that D, with fosses, or ditches, between and beyond them. The outer bank stands around four metres high, considerably more imposing than the inner bank at one and a half metres. The external fosse reaches three and a half metres deep. A causeway carries the entrance across both fosses at the south-east, the kind of controlled access point designed to channel movement and, presumably, to make uninvited entry considerably less appealing.