What had once been a double promontory fort, its innermost bank carried across a natural stone arch to command two adjacent headlands simultaneously, became something more ambiguous: a single defended spur projecting into St Finan's Bay, with the ghost of its broader design visible only on an old Ordnance Survey map and in the published observations of an early twentieth-century antiquarian.

The site is known locally as Doon or An Dún, and was recorded by T. J. Westropp in 1912 under the name Duncanuig. A promontory fort is exactly what it sounds like: a headland cut off from the mainland by one or more artificial banks and ditches, using the sea itself as a natural defence on the remaining sides. At this example, the defences are arranged in a sequence of three banks separated by fosses, the term used for the ditches dug to create them, crossing the promontory roughly at its midpoint. Westropp documented the site in considerable detail, noting a pair of stone gate-piers flanking the entrance causeway, a drystone wall along the outer bank, and traces of a hut in the depression between the outer and innermost banks. None of these features survived to be confirmed by later survey. What does remain is the underlying earthwork structure: the outermost bank curving inward on either side of the causeway entrance, the flat-bottomed fosse to the north-west, the middle bank of earth and stone, and the innermost bank rising to 2.5 metres above the inner fosse. The interior of the fort, roughly 34 metres by 21 metres, is level ground. Where the innermost bank once continued south-eastward across the sea-arch to a second, narrower promontory, there is now only open air and the sound of the Atlantic below.