Recorded on old plans with the modest label 'hut', it sits on a natural rocky outcrop at the extreme western end of the fort's platform, and yet the scholar who drew it seems never quite to have believed it was a hut at all.

The antiquarian Thomas Johnson Westropp surveyed Doon in detail and marked the small semicircular feature with the letter 'e' on his plan. A promontory fort, for those unfamiliar with the type, is a coastal defensive enclosure that uses the natural advantage of a headland, typically sealing off the landward side with one or more banks and ditches. Within Doon's platform, Westropp recorded two other structures alongside this feature, but it was the rocky outcrop at the western end that drew his closest attention. In his 1914 description, he argued that the rock itself had been artificially shaped into what he called a 'rock turret', and that this natural formation had then been reinforced with a curved loop of wall somewhere between 0.9 and 1.2 metres thick, forming a kind of bastion overlooking the access path running along its western side. His interpretation was less domestic shelter and more deliberate control point, a position from which movement into the fort could be watched and challenged. Whatever its original purpose, the physical evidence has largely vanished. No clear surface trace of that curving wall survives today on the grassy top of the outcrop, leaving Westropp's careful drawings as the main record of something that was already ambiguous when he first set it down on paper.