The site at Castledown is a rath, a type of ringfort formed from an earthen bank and, originally, an external ditch called a fosse, which would have enclosed a farmstead during the early medieval period, roughly between the fifth and twelfth centuries. Thousands of these monuments survive across Ireland, but few have been quite so thoroughly domesticated. The interior, measuring roughly 29.5 metres northeast to southwest and 27.5 metres northwest to southeast, is tilled annually, its ancient enclosure now quietly repurposed for growing food, which is, in a way, not so far removed from its original function.

The site sits on a prominent rise in gently undulating pasture, a position that would have offered its early inhabitants clear views of the surrounding landscape, useful both for keeping watch and for signalling social status, since elevated ringforts are often associated with higher-ranking households. By 1913, the monument was recorded on the revised Ordnance Survey 25-inch map as a circular earthwork. When it was formally described in 1970, the bank had been much reduced, surviving largely as a low scarp, best preserved along the southern to western and northeastern to east-southeastern arcs. No trace of an external fosse remained visible. A gap on the east-southeast side, roughly 4.5 metres wide at the top and 1.8 metres at the base, may represent the original entrance. A second gap at the west-northwest, of similar dimensions, is closed by a gate and appears to be a modern addition. The internal and external faces of the bank had also been steepened in relatively recent times, suggesting ongoing intervention rather than simple neglect.

The earthwork remains clearly legible as a circular form in aerial photography, its outline holding firm despite centuries of agricultural pressure. The interior carries a gentle slope facing south-southwest, which likely contributes to its continued use as growing ground, that aspect catching the best of whatever sun Westmeath offers.