This four-storey tower house, with an additional fifth-floor mural chamber and cap-house at its southwest corner, dates back to at least 1640 when it was owned by John St. John, recorded as an ‘Irish Papist’ in the Civil Survey of 1654-56. By that time, the castle was already described as ‘a small castle wanting repaire’, suggesting it had seen better days even then. The structure, built from brown stone and limestone with carefully dressed quoins, measures roughly 8 by 10 metres externally, with walls about 1.4 metres thick.

The castle’s main entrance, a pointed doorway on the eastern wall, leads into a small lobby complete with a murder-hole overhead; a chilling reminder of the defensive mindset of its builders. The ground floor chamber connects via a straight staircase that rises through the building’s southeast corner. The first floor features a vaulted ceiling and, rather unusually, has a garderobe built directly into the vault at the north wall. As you ascend, the second floor reveals the castle’s more comfortable side, with four ogee-headed windows providing light and ventilation. This level also housed a fireplace in the north wall, though only traces remain today. Hidden beneath a trapdoor in one of the window embrasures lies an oubliette; a grim prison cell measuring just over 2 metres long and less than a metre high, where unfortunate prisoners would have been confined in darkness.

The defensive features throughout Mortlestown Castle tell a story of uncertain times: murder-holes protecting the staircases, a yett-hole beside the main door for securing it with an iron grate, and machicolations above the entrance from which defenders could rain down projectiles on attackers below. The upper floors, accessed via a spiral staircase in the southwest corner, would have provided living quarters with better views and security. Though the original parapet no longer survives, a 19th-century bell-cote was added at parapet level, ingeniously incorporating the head of a two-light ogee window from elsewhere in the structure. Today, this tower house stands as a remarkable example of late medieval defensive architecture, its thick walls and carefully planned defences speaking to an era when a home needed to be a fortress.