A promontory fort is an enclosure that uses a natural headland, defended on the landward side by one or more banks and ditches, with the sea cliffs doing the rest of the work. The structure known as Doon is one such place, and sitting roughly midway along its interior platform is this worn oval foundation, measuring around 5.1 metres on its longer north-northeast to south-southwest axis and 5.7 metres across.

The antiquarian T. J. Westropp visited and recorded the site in the early twentieth century, describing it at different points as "an oval house" and "an early hut with rounded corners", language that suggests something domestic and relatively modest rather than ceremonial. What survives now is the barely perceptible line of a delimiting bank, nowhere more than 0.35 metres high, pressed flat under deep sward. Three gaps in that line are discernible at the west-northwest, south-southwest, and east-southeast, and the east-southeast gap, around 1.5 metres wide, is considered the likely site of the original entrance. The building sits only three metres north of a second house within the same fort, which raises quiet questions about how the interior of Doon was once organised and occupied, though the evidence left above ground is too degraded to say much with confidence.

The uneven footing inside the foundations is one of the few things a careful observer might notice on the ground, a slight hollow and irregularity where the interior once had walls around it. The deep grassy covering that obscures the bank is the same across the whole fort interior, making much of the archaeology here a matter of knowing where to look rather than what to see.