That narrow throat is where someone, at some point in the medieval period, decided to build a wall and a gatehouse, turning the promontory into a fort. A promontory fort works by letting the sea do most of the defensive work; the builders only had to seal the one side where attackers could walk on. What makes Doon Point unusual is not just the fort itself but the elaborateness of what was added to it later: a compact, multi-roomed gatehouse inserted into the wall, fitted with multiple drawbar sockets, a lobby roofed with massive pitched slabs, flanking chambers with narrow loops set in splayed embrasures, and an upper storey accessible by an internal stairway. One of the large roof slabs in the lobby has a circular perforation roughly 16 centimetres across, the purpose of which is not recorded.

The defending wall runs east to west across the promontory's neck for 35.5 metres, then turns south at its eastern end to follow the cliff edge for a further 10.5 metres. It stands up to nearly 3 metres high in places and is between 1.5 and 1.9 metres thick, built around a rubble core bonded in clay and gravel mortar. The outer face was later repointed with a mortar containing gravel and shell, and the antiquarian T. J. Westropp, examining the site in 1912, concluded that this outer face was strengthened and the wall slightly raised when the gatehouse was inserted. Just 1.8 metres west of the gatehouse is a second, smaller entrance, only a metre wide, above which Westropp identified projecting stones he interpreted as the remains of a battlemented machicolation, a projecting parapet from which defenders could drop stones or other materials on anyone trying to force the gate below. Inside the promontory, 14 metres south of the gatehouse, the sod-covered foundations of a large rectangular house survive, measuring 12 by 7 metres externally, with walls still standing to an average internal height of about 70 centimetres. Three upright slabs set in a north-south line roughly 3 metres southwest of the gatehouse remain unexplained.