A souterrain is an underground stone-lined passage or chamber associated with early medieval ringforts, typically interpreted as a place of refuge, storage, or both. What makes this one unusual is not so much its construction as its end: the chamber was deliberately packed full of charred material and burnt stone while the ringfort above it was still in use. Someone filled it in, then, at a later point, tried to dig it out again.

Excavation work published by Monk in 1995 uncovered the western portion of the chamber. The part revealed measured at least 2.9 metres east to west and reached a maximum depth of 1.85 metres, with stone-built north and south walls that were slightly corbelled, meaning the stones were laid so as to project inward and upward, narrowing the space above in a rudimentary arch. The western wall was simply earth-cut, and the chamber was entered by earth-cut steps at that end. The floor widened as it ran eastward and then began to slope upwards near the excavation limit, suggesting the full extent of the structure was never uncovered. No roofing slabs were found in place; instead, a layer of burnt material near the base of the chamber was interpreted by the excavator as the carbonised remains of a timber roof. That detail alone sets it apart from many souterrains, which were roofed with large flat stone lintels. Underneath all of this, the souterrain was found to cut through the foundation trench of an even earlier circular house, placing it within a sequence of occupation at the site stretching back before the ringfort itself was built.