This rectangular tower house, built from roughly coursed limestone rubble with carefully cut corner stones, once stood three storeys high with its 53 feet length and 23 feet breadth dominating the streetscape. Today, only the lower portion of the east gable survives to first floor level, rising between 6.5 and 8 metres, whilst the outlines of the other walls remain visible at ground level. The surviving architecture reveals typical tower house features: a splayed embrasure for a small window with hood moulding, wall cupboards, the remnants of a spiral staircase in the northeast corner, and a fireplace whose opening has been narrowed over time.

The castle's most dramatic chapter unfolded during the Cromwellian campaign in Ireland, when Captain Marcus Geoghegan of Westmeath made his stand here with his company in what contemporary accounts described as "a petty castle, a kinde of gate hous, thatched with strawe". Local tradition preserves a grim tale of the siege's end; Cromwellian forces allegedly reduced the castle by scalding its defenders to death, along with townspeople who had sought refuge in the lower rooms. This haunting story gained credence in 1830 when workmen levelling the garden behind the castle uncovered an immense quantity of bones, estimated to be the remains of over 150 people.

Historical records paint a picture of the castle's varied fortunes. The Civil Survey of 1654-6 lists it as a slated house with castle on West Street, owned by Richard Foster, an Irish Catholic, and tenanted by Protestant Patrick Vance; a telling detail of the property transfers that followed Cromwell's conquest. By the early 19th century, Robertson's sketches show it as a substantial but roofless ruin, one of four tower houses visible from St Augustine's Well. The 25-inch Ordnance Survey map would later annotate it simply as "Castle (in Ruins)", whilst recent archaeological work has uncovered evidence of a blocked doorway in a nearby boundary wall, possibly linking the castle to its former gardens.