Whoever constructed what is now known as Doon Eask, or An Dún, was not content to rely on nature alone. At the summit of the ridge, some 165 metres above sea level, the remains of a curving wall once sealed off the approach from the landward side. Below that, across the narrow neck of the promontory, a second and far more elaborate system of earthworks was laid out: three banks and fosses, the term fosse referring simply to a defensive ditch, extending 45 metres across the slope, with a 2.2-metre causeway threading through them close to the western cliff edge. The arrangement is deliberately asymmetrical, the banks on the west side positioned opposite the ditches on the east, which would have made any approach along the causeway an awkward and exposed business.

T. J. Westropp described the site in 1910, by which point the upper wall still stood between roughly 0.9 and 1.2 metres high. Today its facing stones survive to only 0.7 metres. The lower defences are more substantial: the innermost fosse, cut directly into the rock, rises 3.5 metres to the wall that forms the final barrier, which is built partly from a natural rock ridge and partly from drystone courses, at least three of which remain near the causeway. Inside the fort, the ground climbs in natural terraces to a rocky pinnacle at the southern end. Partway down the seaward slope, a row of upright slabs runs east to west; they may have been placed deliberately, though the possibility that they simply broke away from the surrounding rock faces cannot be ruled out. Below them, stone-faced walling with slight depressions at either end may mark the positions of former hut-sites. A burial ground, Relig an Dúna, has been recorded on this same southern face, and the upright slabs may be connected to it. Two flat slabs near the outer fosse were once labelled on Ordnance Survey maps as a Giants Grave, but scholars have found no evidence that they ever formed part of a megalithic tomb; locally they are known as Leaba na bhFiann, the Bed of the Fianna.