A drumlin, for the unfamiliar, is one of the smooth, elongated hills left behind by glacial activity, and they are everywhere in this part of Ulster, rolling across the countryside in clusters. That someone chose this particular slope, just shy of the summit, to construct a defended enclosure says something about how prehistoric and early medieval communities read the land, favouring elevated positions that offered both outlook and a degree of natural protection without full exposure to the ridge.

The fort at Listellan is roughly circular, measuring about 35 metres across at its widest, and is defined by an inner earthen bank separated from an outer bank by a fosse, the term for a defensive ditch dug to reinforce a rampart. The inner bank, still standing around 0.8 metres high, has a causeway entrance to the east, the causeway measuring over four metres wide at the top, suggesting a deliberate and formal approach. The outer bank complicates things a little. There is a slight berm, a narrow flat shelf, between the fosse and the outer bank along its northern and eastern stretches, and on the north-west to east side the outer bank has been incorporated into a later field boundary with stone facing added to its exterior. These details suggest the outer bank may not be original to the fort at all, but a later addition or modification, possibly nothing older than a field enclosure from the post-medieval period. What remains of the interior is now heavily overgrown, the whole area having reverted to the kind of tangled vegetation that slowly reclaims earthworks when farming no longer requires their removal.