The more visible feature today is a fosse, the defensive ditch cut across the neck of the promontory to seal it off from the mainland, a standard arrangement for this class of coastal fort. It has silted up considerably since it was last in active use, measuring roughly two metres wide and one metre deep now, but the antiquarian Thomas Westropp, writing in 1914, recorded it as cut into the bedrock to a depth of six feet and a width of twelve, which gives a clearer sense of the original effort involved.

Westropp also noted traces of walling running along the edge of the promontory, and he interpreted these as belonging to the earliest phase of defences on the site, predating the castle whose sod-covered remains sit just behind the fosse. That reading is complicated by a comparison with a similar site at Dooneendermotmore, elsewhere in County Cork, where comparable walling turned out to be late medieval in date rather than prehistoric. The question of whether the Dunnycove walling is genuinely ancient or simply an earlier episode of medieval construction remains open. The castle itself, now largely buried under turf, adds a further layer; the promontory appears to have been reused and refortified across a long span of time, each phase making use of the natural defensibility of the ground without necessarily erasing what came before.