At its tip, cut and shaped from living rock, sits a promontory fort, though not the coastal cliff-edge variety most people picture when they hear the term. This is an inland example, occupying a lake peninsula rather than a headland above the sea, which makes it comparatively rare and quietly instructive about just how versatile the promontory fort as a form really was.

A promontory fort is essentially a defended enclosure that uses natural topography, a cliff edge, a steep-sided ridge, or in this case a lakeshore peninsula, to reduce the amount of artificial fortification required. Here, the builders did considerable work regardless. The main enclosure sits at the south-eastern end of the peninsula on a roughly rectangular flat-topped rock outcrop, approximately 32 metres by 24 metres, whose edges were cut and shaped into near-vertical scarps rising between two and four metres. Steps roughly quarried from the exposed rock give access through the original entrance on the northern side, flanked by a single row of large irregular blocks laid at the base of the scarp. Inside, slightly south of centre, a roughly circular flat-topped mound survives, defined by its own earth and stone scarp nearly two metres high. Its original purpose is unrecorded. Running across the peninsula about eight and a half metres further north are the stone footings of a drystone wall, forty metres long, with a shallow fosse, a defensive ditch, cut along its northern face. A gap in the wall paired with a causeway over the fosse marks where a second entrance once stood, giving the whole arrangement a layered quality, an outer line of wall and ditch, then the main rock platform beyond.

Perhaps the most intriguing element lies fifteen metres north of that outer wall. A broad U-shaped depression, open toward the lake at its western end with a base that slopes gently to the water, has been interpreted as a possible boat slip or small harbour. Immediately beside it, a roughly rectangular area defined by rubble blocks and apparently surfaced with stone adds to the impression of deliberate infrastructure. Taken together, the defended platform, the outer wall and fosse, and the possible landing place suggest a site that was managing both land access and water access, which would make considerable sense for anyone seeking to control or exploit movement along Lough Gill itself.