What remains today is a long rectangular structure, roughly 12.2 metres east to west and 21 metres north to south, with walls nearly two metres thick built from locally quarried limestone rubble. Though time has not been kind to this once three-storey fortification, enough survives to reveal its unusual transitional design, which combines elements of both traditional tower houses and the later fortified houses that became popular in Ireland during the 16th and 17th centuries.

The castle’s ground floor originally featured a large northern chamber with a wooden ceiling, alongside two barrel-vaulted rooms running north to south, each measuring about 2.6 by 5.8 metres. These vaulted chambers, which still show traces of the wicker centring used in their construction, are connected by a large arched opening in their dividing wall. At the southern end of the building, a now blocked chamber once provided access to a garderobe tower at the southeast corner and possibly to another tower at the southwest angle, though the latter has been completely destroyed. The first floor housed what appears to have been the main hall, positioned above the vaulted chambers and northern room, with mural stairs built into the walls leading to the now vanished second floor.

Surrounding the castle are the remnants of a circular bawn wall, best preserved on the southern side where the roughly coursed limestone construction is still visible. An interior offset halfway up the wall likely supported a wooden defensive platform, whilst traces of a defensive ditch or fosse can be detected around the base of the fortification. A three-metre gap on the southwest side, possibly crossed by a causeway, marks what may have been the original entrance to this intriguing fortress. Despite its ruined state, Castletown Castle offers valuable insights into a period when Irish castle design was shifting from purely defensive structures to more comfortable, albeit still fortified, residences.