The site belongs to a category known as a ring-ditch, the buried remains of a circular ditch that once enclosed a prehistoric structure or burial mound. Over centuries of farming, the earthwork itself has been levelled entirely, but the ditch fill, with its different soil composition and moisture retention, causes the crops growing above it to respond differently to drought. The result is a cropmark, a faint but readable ghost of the original feature that becomes legible only when viewed from altitude under the right conditions.

The enclosure at Castletown was recorded on an aerial photograph taken as part of the Cambridge University Collection of Aerial Photography, catalogued under reference BDI 90. On the ground, in what is now a ploughed field set within a level, low-lying landscape, there is nothing to see. The site neither projects above the surface nor leaves any visible trace in the soil as a walker would encounter it. Its existence is known entirely through that single aerial image, which captures in faded tones what would otherwise be a completely invisible piece of the county's prehistoric past. Ring-ditches of this kind are often associated with Bronze Age funerary monuments, the ploughed-flat remnants of barrows or ring cairns, though without excavation the precise date and function of any individual example remains uncertain.