The site is a promontory fort, a type of coastal stronghold in which a naturally projecting headland was fortified across its narrowest point, using the sea itself as the primary barrier on the remaining sides. What survives here is that cross-cutting ditch, or fosse, hewn directly into the rock: it runs about twenty-seven metres north to south, reaches nearly eighteen metres across at its widest point, and drops to a depth of three metres. The outer edge is noticeably ragged, particularly along the southern side, suggesting either the uneven character of the underlying geology or the wearing away of what may once have been a more deliberate profile.

The ridge itself is slender, between eight and thirteen and a half metres wide, which gives a sense of just how exposed and marginal a position this was. Whether the site dates to the Iron Age or to an earlier period is not recorded, but promontory forts of this kind are found along Ireland's Atlantic coastline in considerable numbers, and Dunmanus Bay has the kind of rugged, sheltered-yet-exposed geography that made such positions attractive to communities who depended on both land and sea. Immediately to the west of the fosse stands Dunbeacon Castle, a later structure that suggests the same narrow ridge continued to hold strategic value long after the original fort had fallen out of use.