Somewhere in the fields of Ballysax Great, a monument that once rose two metres above a low Kildare ridge has entirely vanished. Not ruined, not reduced to a scatter of stones, but gone, leaving no visible surface trace whatsoever. The speed of its disappearance is quietly startling: measured and described in 1955, gone by 1972.

The first Ordnance Survey six-inch map of 1838 recorded it as a circular enclosure roughly forty metres across and labelled it a tumulus, the Latin term cartographers of the period used for burial mounds of uncertain date. By the time the revised OS map was produced between 1939 and 1940, the feature had shrunk in recorded diameter to around thirty metres, with a field boundary wrapping around its base from the south-west through north to east. A 1955 description captured it at its most detailed: a raised circular platform two metres high and thirty-one metres across, with a smaller secondary mound at its centre rising to about eighty centimetres. That description also placed it on a low ridge, which would have given even a modest earthwork a degree of visual presence in the landscape. Seventeen years later, nothing remained to see. Whether agricultural levelling or some other intervention was responsible, the record does not say.

What the mound actually was remains an open question. It may have been a motte, an earthen castle mound of the kind introduced by Anglo-Norman settlers, typically a steep flat-topped hill of compacted earth designed to carry a timber tower. The possibility gains some weight from a documentary reference to a castle at Ballysax in 1307, though its precise location has never been established. Whether the mound and the castle were one and the same, adjacent to each other, or entirely unrelated is something the surviving evidence cannot resolve.