Historical records from that year show it belonged to William and Redmund mac William Burke, members of one of Connacht’s most prominent Norman-Irish families. The castle forms an almost perfect square, measuring 6.8 metres long by 6.2 metres wide, and rises four storeys high; a compact but formidable defensive residence typical of late medieval Ireland.

The tower’s architecture reveals centuries of modifications and practical adaptations. The original entrance, centrally positioned in the north wall, was later expanded into a larger archway, with a matching arch added to the south wall, though this has since been blocked up. Inside, a spiral staircase tucked into the northwest corner provided access between floors, whilst sturdy stone vaults separated the ground from the first floor and the second from the third. Between these upper levels, an intramural passage runs along the western wall, likely leading to a garderobe, or medieval toilet. The windows throughout display a variety of styles; single and double lights with ogival, round, and flat heads, suggesting different phases of construction or repair over the centuries.

Today, only the base of the rooftop parapet remains, and evidence of a later pitched roof can be seen on the southern wall, probably dating from the same period as the enlarged archways. The castle has accumulated various additions over time, including the ruins of a 19th-century house built directly against its western wall. To the north, rectangular foundations mark the site of a second house, whilst traces of what may have been the original bawn, or fortified courtyard, can still be detected in the field to the northeast. Despite these later intrusions and the passage of more than four centuries, Killeen Castle remains an impressive example of the tower houses that once dotted the Irish landscape, serving as both home and fortress to the country’s medieval elite.