What actually lies here at Lugdoon is a souterrain, an underground or semi-underground stone-built passage and chamber used in early medieval Ireland typically for storage, refuge, or both, and it sits tucked within the interior of a cashel, a circular stone-walled enclosure of the same broad period.

The structure is positioned in the south-western quadrant of the cashel, close against the inner face of the enclosing stone bank. A limestone lintel slab marks the entrance, and beyond it opens a small chamber built in drystone construction, meaning the stones are laid without mortar, relying entirely on careful placement for their stability. Off this chamber runs a creep-hole, a deliberately narrow connecting passage designed so that a person would have to squeeze through on hands and knees, oriented towards the east-north-east. The tightness of a creep-hole is not accidental; it slows pursuit and makes the interior easier to defend or conceal. That this feature survives at all within an intact cashel setting makes Lugdoon a quietly unusual site, with two complementary early medieval structures occupying the same ground.