A souterrain is an underground passage, typically stone-lined and roofed with flat lintels, built during the early medieval period as a place of refuge or cold storage. Here, the structure has lost its roof, and what survives is a flat-bottomed hollow nearly twenty metres long, just over two metres wide, and less than a metre deep in places, its deliberate angular turns still legible despite centuries of collapse.

The Z-shape is not accidental. Souterrains were often designed with bends and narrowings precisely to slow down anyone forcing their way inside, turning a simple passage into a defensive obstacle. This one begins just south of the ringfort's centre, runs south for five metres, pivots sharply eastward for seven metres, then turns south again for a final stretch of seven and a half metres. Traces of drystone walling appear intermittently along its course, and several collapsed roof lintels remain visible among the debris. A local record from 1914 by a writer named Neary noted that the enclosure's garth, meaning its interior yard, contained a souterrain, suggesting the feature was recognised and described over a century ago, though the passage had already lost its integrity by then.