The castle sits on low-lying pastureland, with Lough Derravaragh just over a kilometre to the southwest and bogland roughly two kilometres to the west. Though no castle appears on the 1657 Down Survey map of Mayne parish, historical records from 1641 identify Thomas Fitzsyman as the chief proprietor of Williamstown lands. By the time of the 1837 Ordnance Survey, the structure was already marked as ‘Castle’, and by 1911 it had been relegated to ‘Castle (in Ruins)’.

What remains today is a poorly preserved square tower house, originally measuring about eight metres east to west with walls a metre thick, now standing approximately five metres high. Built from undressed limestone blocks laid in regular mortared courses with rounded corner stones, only the north wall survives intact, along with small sections of the eastern and western walls. The ground floor once featured a stone vaulted ceiling, traces of which can still be seen springing from the inner face of the north wall, beneath which sits a narrow, flat-headed loop or window. Evidence suggests a stairway once occupied the northwest corner, providing access to the upper floors.

The first floor reveals more architectural details despite the damage; a large, broken-out window dominates the north wall, whilst blocked window openings are visible in both the east and west walls. The southern half of the castle has been completely levelled, and post-1700 farm buildings have been constructed against the external face of the west wall, giving the ruins a curious blend of medieval and more recent agricultural architecture. No traces of a bawn wall, which typically would have enclosed and protected such tower houses, remain visible today.