It has carried the name Nangle's Castle on Ordnance Survey maps since at least 1836, yet that association with the Nangle family may be no more than cartographic habit. Equally plausible is that the building formed part of the adjoining Augustinian Priory complex, whose north wall runs immediately to the south. The ambiguity is unresolved, and the ruin sits quietly with it, pressed between religious and domestic medieval Trim, about eighty metres north of the River Boyne.

The Nangle family arrived in Meath with the Norman settlement and made themselves consequential quickly. Hugh de Lacy granted the barony of Morgallion to Gilbert de Nangle, as recorded in the twelfth-century verse chronicle known as the Song of Dermot and the Earl. By around 1400 the family were styling themselves Barons of Navan and appearing on lists of Keepers of the Peace, a role that amounted to a form of local judicial authority. Whether they had anything to do with this particular building remains uncertain. What survives of it is modest: a rectangular structure measuring roughly 17 metres east to west and 7 metres north to south, with only the east and west walls still standing to anything approaching first-floor level. The ground floor is plain, with no evidence of vaulting, but the first floor retains two more distinctive features: a garderobe, the small latrine recess common in medieval buildings, at the north-east angle, and a corbelled overhanging chamber at the south-east angle, partly carried by the wall of an adjacent building. That corbelled projection, jutting outward on bracketed stonework, suggests a structure of some architectural ambition, whatever its original purpose.

The building sits within a dense archaeological landscape. Excavations in the immediate vicinity have uncovered house remains, medieval pits and deposits, and post-medieval structures along High Street to the north. In 2008, unauthorised groundworks about sixty metres to the north-east, on two burgage plots near the Yellow Steeple, brought archaeologists in to assess the damage. Burgage plots were the long, narrow urban landholdings typical of medieval planned towns, and the exposed sections here yielded animal bone-filled pits and the traces of ditches, though dating evidence proved elusive. The cumulative picture is of a medieval town that has left considerable archaeology just beneath its surface, with this weathered, name-uncertain building somewhere near the middle of it.