What began as a rectangular hall-keep in medieval times was transformed into a three-storey tower house during the fifteenth century, reflecting the changing defensive and residential needs of its occupants. The Civil Survey of 1654-6 recorded it as 'an old ruined Castle & bawne & three cottages', part of the Manor and Lands of Bellagh Caple, which in 1640 belonged to various proprietors including Richard Butler of Kilcash and members of the Kennedy family.

The castle's construction tells a fascinating story of medieval building techniques and later modifications. Built from roughly coursed limestone with walls nearly two metres thick, the structure measures approximately 13 metres north to south and 9.5 metres east to west. Its most distinctive feature is the double barrel vault over the ground floor, divided into two chambers by a rubble wall, with the southern chamber's vault still showing evidence of the plank centring used during its construction. A spiral staircase, contained within an angle tower projecting from the southeast corner, provides access to the upper floors, whilst various architectural details reveal the building's complex history; blocked doorways, inserted windows, and multiple phases of entrances, including a fifteenth-century pointed doorway that was later replaced in the late sixteenth century with another featuring punch-tooling and drafting.

The interior spaces reflect both defensive considerations and domestic comfort. The first floor, entered through a pointed doorway from the spiral stairs, contains a large fireplace in the west wall and storage cupboards, whilst evidence of a blocked trefoil or cusped ogee-headed window speaks to the castle's more refined architectural elements. The second floor appears to be a later insertion, complete with an open fireplace in the south wall and joist holes for wooden flooring. Local tradition maintains that remnants of the original bawn wall survive to the southwest, with some sections incorporated into nearby buildings, including a house whose southern gable contains part of this defensive perimeter. Though time has taken its toll on the structure, with the northern chamber's vault largely collapsed, Ballycapple Castle remains a remarkable example of how Irish tower houses evolved from simple defensive structures into complex residences that balanced security with comfort.