That kind of deliberate, careful stonework suggests something more considered than a simple enclosure thrown up in haste. The site sits at roughly 118 metres across in both directions, shaped, as the antiquarian Thomas Johnson Westropp noted during several visits between 1897 and 1902, like a pear, with wide views opening out across the Burren to the south-west and east.

Westropp's observations remain the most detailed on record. He described a well-cut fosse, which is a defensive ditch dug around the outside of the bank, and identified three distinct entrances. Two lay to the west; one of these may have been crossed by a causeway mound, while the other, some eight metres wide, had a sunken central section and what Westropp interpreted as a square-cut projection of shale that could once have supported a plank or drawbridge. The eastern entrance was the one reached by the seven steps, of which a single step of about sixty centimetres remains visible today at the end of the fosse. The fort itself is classified as a cashel, a term generally used for a stone-walled enclosure of early medieval date, though here the defences are largely earthen, with only some remnant stone facing surviving. Westropp, writing in 1915, placed it within a broader linear arrangement of similar forts extending westward from Kilfenora, and described most of these, Doon included, as comparatively featureless from a structural point of view. What sets it apart is not grand stonework but the precision evident in specific details: the fosse at the west is flat-bottomed and well-preserved, and the shale bedrock at the east was cut in antiquity to form a deliberate horizontal ledge, suggesting the builders worked carefully with the natural geology rather than simply piling material on top of it.

The interior, measuring roughly 82 metres north to south and 99 metres east to west between the crest lines, is divided by a later field bank running north to south, and much of it is overgrown, particularly to the east of that bank. A modern entrance track now crosses the bank at the eastern side, roughly where the old stepped entrance once led visitors in. The fosse is best examined at the western arc, where its flat base and external depth are most clearly legible beneath the vegetation.