Built around 1550, this square-plan fortress features distinctive architectural elements including a circular bartizan with gunloops on its south-east corner, carved limestone chimneystack to the north-west, and a turret housing the staircase. The structure measures approximately 9 metres long by 7.5 metres wide, with rubble limestone walls reinforced by cut and tooled quoins. Though substantially modernised during the 19th century by the Croker family, who added paired and single light windows with cut limestone surrounds, the castle retains many of its defensive features, including loop openings and a square guardrobe opening at ground level on the east elevation.

The castle's documented history reveals a complex succession of ownership typical of Irish fortified houses during the tumultuous 16th and 17th centuries. In 1625, Theobald or Tibbott Bourke held Williamstown Castle, though notably excluding the "Geist Hall", whilst by 1636 his son John occupied only a room within the castle, along with the bawn and a stone house in the northern section. The 1654-56 Civil Survey confirms Theobald Lord Baron of Brittas, identified as an Irish Papist, as owner of the property which included both castle and bawn. Following the Cromwellian period, ownership became disputed; in 1655 Lord Brittas, Dr T. Arthur and others all claimed the castle and bawn, with the property eventually being granted in 1666 to Colonel Clayton and W. Matthews, though the middle storey was confirmed to Dr Arthur's daughters.

Archaeological classification places Williamstown Castle as a Type 4 tower house, representing the later evolution of Irish defensive architecture from the late 16th or 17th century. These sophisticated structures incorporated advanced defensive features such as tourelles with shot holes, hood-moulded and mullioned windows, parapet water spouts and cruciform roofs, marking a transition between medieval fortifications and more comfortable domestic dwellings. The site also includes 19th-century outbuildings with pitched corrugated iron roofs and rubble limestone walls featuring red brick voussoirs, whilst dressed limestone boundary walls with stepped crenellated copings and square-profile piers frame the property's entrance. Interestingly, the castle appears on different townlands depending on the map consulted; the 1840 OSi 6-inch map places it in Williamstown townland, whilst the revised Cassini OSi 6-inch map shows it standing in Ballynagarde.