The souterrain, an underground stone-lined passage and chamber of the sort typically associated with early medieval ringforts and used for storage or refuge, had been entered by excavators in 1989 under the direction of archaeologist Anne-Marie Lennon. The work was carried out ahead of partial destruction of the site by a Sandoz factory, meaning the investigation was urgent and the window to learn anything was narrow.

The souterrain itself is a compact but carefully constructed space. A creepway roughly four metres long and less than a metre in both width and height leads from the eastern side of the ringfort interior, angling downward toward the north-north-east. At its far end sits a single oval chamber built from dry-stone walling, corbelled inward from a height of 1.2 metres, a technique in which courses of stone are progressively angled to close the roof, meeting finally under three large capstones. The chamber measures about two metres by 1.5 metres and sits nearly two metres below the surface. The creepway itself had been deliberately backfilled with stones, which would have effectively sealed the chamber from use or access. Inside the chamber, excavators found the almost complete skeleton of a dog. Whether the animal was placed there as some form of deposit, died there by accident after becoming trapped, or represents something else entirely is not recorded. The backfilling of the entrance passage does suggest a deliberate act of closure, which makes the dog's presence all the more quietly unresolved.