It is not especially large, roughly 32 by 23 metres, and much of its stone has been robbed away over the centuries for other purposes. What remains, though, is structurally coherent enough to read as a serious piece of military architecture, complete with artillery bastions projecting at the corners in a spear-like shape, a round-headed entrance gate midway along the south-east wall, musket loops in the western bastion, and a small semicircular turret tucked against the south-west wall. The bastions, which are angled projections designed to allow defenders to cover the faces of adjacent walls, vary noticeably from corner to corner, suggesting the fort was either built in stages or adapted as it went up. Inside, a courtyard is flanked on three sides by ruined buildings, and at its centre is a blocked well.

The fort was built in 1656 and 1657, during the Cromwellian administration of Ireland, when control of Atlantic island harbours carried real strategic value. The structure appears to have incorporated earlier works already on the site from the Confederate period, the decade or so of Irish Catholic Confederation governance that preceded Cromwellian conquest. Before any of that, according to local tradition, the headland was occupied by something called Bosco's Castle, a name that points toward an older, possibly medieval or early modern, fortification whose details are now obscure. The layering of these three phases, Confederate earthworks beneath a Cromwellian fort on what may have been a much earlier defended site, gives the ruin a compressed quality, as if several different ideas about how to hold this particular piece of ground have been stacked on top of one another over time.