What remains at Killowen sits right on the lip of a sheer cliff-face, its interior now D-shaped, the other half long since swallowed by erosion and Atlantic weather. When the Ordnance Survey mapped the area in 1842, roughly half the site had already gone. Since then, the process has continued without pause.

The fort was originally a roughly circular enclosure, around 60 metres in diameter. Earthworks of this kind, known as ringforts, were typically used as enclosed farmsteads or defended settlements during the early medieval period in Ireland, though some examples may be considerably older. At Killowen, two earthen banks survive, each standing approximately five metres high, with a flat-bottomed fosse, or ditch, running between them. A fosse is a defensive ditch dug to reinforce the banks on either side; here, the external fosse has been infilled over time, though its outline can still be read in the exposed cliff-face. The antiquarian Thomas Westropp recorded the site in 1914, and even then noted it as eroded. The contrast between what he saw and what exists now gives some sense of the pace of loss over the intervening century.

The cliff edge continues to give way, so the geometry of the surviving earthworks shifts with each winter. The section visible in the cliff profile, where the old fosse appears in cross-section, offers an unusually clear view of how the fort was constructed in layers, something normally hidden beneath the ground.