This is a souterrain, a type of subterranean structure built during the early medieval period, typically in association with ringforts, and used variously for storage, refuge, or concealment. What makes the Grange example quietly remarkable is its T-shaped plan, a layout that required considerable forethought from the people who built it, and the precision with which it has endured.

The structure is drystone-built, meaning its walls were constructed without mortar, relying entirely on the careful placement of stone to hold its form over centuries. It consists of two chambers linked by a creep, a deliberately constricted connecting passage that would force anyone moving between chambers to crouch or crawl, slowing down an intruder while a defender held the advantage. The first chamber runs roughly north-north-west to south-south-east and measures 7.7 metres in length. About five metres from its northern end, a short passage on the north-east side leads into the creep, which at just 0.63 metres long and 0.37 metres wide is barely wide enough for a person to pass through. Beyond it lies the second chamber, running west-south-west to east-north-east and measuring approximately ten metres long and three metres wide, though this section is currently inaccessible. Entry to the first chamber is possible via a displaced roof lintel at its southern end, the stone having shifted from its original position but leaving a gap large enough to admit a person.