The oval earthwork platform at Doon, measuring roughly sixty metres east to west and fifty metres north to south, is defined only by a scarp, a low edge or slope where the ground drops away to mark a former boundary. Whatever structure once stood here has been reduced to that faint outline, and the name "fort" now carries more weight than the visible archaeology can easily support.

The site is further complicated by its associations. A castle is recorded alongside the earthwork, suggesting the hillock was reused or reoccupied across different periods, as was common in Ireland where a naturally defensible rise attracted settlement generation after generation. More intriguing still is the local tradition of a cave somewhere on or near the hill. This is thought to be a possible souterrain, an underground stone-lined passage typically built during the early medieval period, often for storage or refuge, sometimes connecting to a nearby ringfort or enclosure. No surface trace of it survives, which means it either collapsed long ago, was deliberately concealed, or the memory of it has drifted slightly from its actual location over the centuries. Oral tradition has a way of preserving knowledge that the ground itself no longer shows.