Just 55 metres from Williamstown House, this once imposing structure appeared as a small square building marked 'Castle in ruins' on the 1837 Ordnance Survey map. By the time surveyors returned in 1911, nothing remained but a notation reading 'Castle (Site of)', indicating the castle had been completely levelled during those intervening decades.

Though the castle itself has disappeared, careful observation reveals traces of its former defences. The structure once stood in the southern quarter of a large rectangular enclosure, surrounded by an earthen bank and external fosse; likely the remnants of a castle bawn, the fortified courtyard typical of Irish tower houses. Today, the townland boundary fence with Williamstown (Rochford) runs just 20 metres to the east of where the castle once stood. A curious relic may have survived the demolition: a stone head now built into an outhouse at Williamstown House farmyard, possibly salvaged from the original castle.

The castle's history proves equally elusive. Neither the 1655-59 Down Survey parish map of Killulagh nor the Barony map of Delvin shows any trace of Williamstown Castle, suggesting it may have already fallen into disrepair by the mid-17th century. What we do know is that in 1641, these lands belonged to Sir Thomas Nugent, recorded in the Down Survey as an 'Irish Papist', placing the estate firmly within the complex religious and political landscape of pre-Cromwellian Ireland.