Built as a rectangular three-storey structure with an attic, the castle measures 10 metres northeast to southwest and 8.5 metres northwest to southeast, with walls 1.4 metres thick. A circular stair turret projects from the western angle, its interior diameter spanning 2.15 metres. The entire structure is constructed from roughly coursed limestone rubble with some sandstone, and patches of the original harling still cling to the exterior walls. A late 17th-century house was later built adjoining the tower’s southwest gable, creating an intriguing architectural timeline.

The Civil Survey of 1654-6 provides a fascinating glimpse into the castle’s past, recording that ‘Kyllosty’ featured ‘a Castle, an Oarchard & Garden’ and was owned by James Brittin of Kyllosky, noted as an ‘Irish Papist’, in 1640. The building’s defensive nature is evident in its gun-loops positioned throughout the stair turret, whilst its residential comforts are shown in the carefully crafted windows; flat-headed and chamfered externally with punch-tooling within drafted margins. The upper floors feature two-light windows with external hood-mouldings where they survive. Inside, wooden floors were supported on corbels and accessed via pointed doorways from the spiral staircase, with each level containing fireplaces for warmth, including one on the second floor with an unusually elaborate carved chamber stop on its western jamb.

The castle has undergone various modifications over the centuries, with 18th or 19th-century alterations including the insertion of fireplaces into existing embrasures. The ground floor’s main chamber, measuring 7.9 by 5.74 metres, was originally entered through a doorway in the southwest wall, though this is now blocked. A large opening in the southeast wall was likely created later to provide access from the yard. The building once featured a bartizan at the eastern angle, of which only the corbel supports remain, and the walls were topped with gables that didn’t extend to the corners; a distinctive architectural feature. Drainage spouts projecting from the walls and the remains of chimney bases recorded in 1909 sketches hint at the castle’s former grandeur, though time and a notable crack running up the centre of the southeast wall serve as reminders of its considerable age.