On the southern end of a north-south ridge in County Waterford, a broad, shallow bowl is scooped into an east-facing slope, ringed by an earthen bank wide enough to walk across. The enclosure measures roughly 90 metres north to south and 80 metres east to west, and where the bank survives in something close to its original condition, on the south-western arc, it runs to about 13.5 metres in width, rising some 2.2 metres on the interior side and 2 metres on the exterior. Those are substantial dimensions for a largely unregarded earthwork sitting quietly in an arable field. Much of the northern, western, and eastern perimeter has been absorbed into modern field boundaries, and the south-eastern stretch has vanished altogether, but the surviving section gives a clear enough sense of the original scale.

Tom Condit and Malachy Gibbons, writing in the County Waterford journal Decies in 1988, identified the monument as henge-type, a classification that carries some weight. A henge is a Neolithic or early Bronze Age ritual enclosure, typically defined by a circular or oval bank with an internal ditch, built not for defence but for ceremony. The dished interior at Castletown, lower than the surrounding bank rather than raised above it, fits that general pattern. Henges are associated predominantly with Britain and Ireland's Atlantic fringe, and while the great examples at Newgrange and the Boyne Valley tend to dominate the conversation, smaller and less celebrated examples like this one are scattered across the country, often surviving only in fragments precisely because centuries of agriculture have worked away at their edges. Here, the interior was under cereal cultivation when the site was surveyed, which is a reminder of how thoroughly these ancient boundaries have been folded into the working landscape.