What appears to be simply a dramatic coastal edge turns out to be a place where defence upon defence has been stacked across centuries, each generation apparently recognising the same strategic logic in the same strip of cliff-bound rock.

A promontory fort is a form of enclosure in which natural cliffs or steep slopes do most of the defensive work, leaving only the landward side to be cut off by earthworks. Here, those earthworks survive in remarkable stratigraphic detail. Directly in front of the curtain wall of the late medieval castle complex at Three Castles Head, earlier defensive lines can still be traced for around 25 metres running from the curtain wall southward toward the cliff. A wide fosse, essentially a rock-cut or dug ditch, runs about 2.5 metres out from that curtain wall, measuring some 8 metres across and 1.6 metres deep. Beyond it sits a bank roughly 1.2 metres high, and then a second, shallower fosse, 4 metres wide and 0.6 metres deep. Further out still, running at an oblique angle to the others, a low bank with a gap flanked by upright slabs was identified by T. J. Westropp in 1915 as the probable remains of the earliest phase of defensive works on the promontory. Westropp was a prolific recorder of Irish monuments in the late nineteenth and early twentieth centuries, and his reading of this outermost feature as the oldest layer is consistent with the general pattern of coastal promontory reuse found elsewhere in Ireland, where Iron Age or early medieval earthworks were later incorporated into or overlaid by medieval masonry fortifications.

The traceable earthworks end at a modern wire fence, which gives a faintly strange quality to the site: several thousand years of accumulated defence, terminated by a fencepost. Visitors approaching Three Castles Head should expect rough coastal terrain, and the remains of the earthworks, being low and grassy rather than dramatic, reward patience and a slow pace along the landward edge of the headland.