The hollow runs roughly north-north-west to south-south-east, measuring just over fourteen and a half metres long and a little over five metres wide, dropping less than a metre below the surrounding surface. Unassuming from any distance, it is the kind of feature that most walkers would cross without a second thought.

The enclosure it sits within is a rath, a type of circular or oval earthen ringfort typically built during the early medieval period in Ireland, used as a defended farmstead. The depression in its south-western quadrant is thought to be the possible remains of a souterrain, an underground passage or chamber often constructed within or close to raths. Souterrains were generally built from stone or timber, roofed over, and used for storage, refuge, or both, their cool interiors well suited to keeping food. Over centuries, the roofing material collapses and the infill of soil gradually levels the structure from within, leaving precisely the kind of linear sunken trace visible here. Whether the Castletown feature is genuinely a souterrain or simply a natural or later disturbance remains uncertain, though its alignment, dimensions, and position within the rath are consistent with the type.