Built by the Rossiter family of Rathmacknee, the castle changed hands over the centuries, passing to Richard Jennings who held it in 1640 along with 265 acres of surrounding lands split between Newcastle and Gaynestown. By the 18th century, ownership had transferred to the Lett family, who likely added the five-bay dwelling house that still clings to the tower’s eastern side.

The compact tower house, measuring roughly 6 metres by 5.5 metres externally, showcases typical defensive features of its era. The original pointed doorway on the east wall, now blocked, once led visitors through a narrow lobby protected by a murder hole above; a sobering reminder of the castle’s defensive purpose. Inside, a clever arrangement of mural and newel stairs connects three storeys, each with distinct characteristics. The ground floor contains double-splayed loops for defence, whilst the first floor sits beneath a barrel vault. The second floor, positioned above this vault, retains one complete granite-framed window with chamfered edges on its south wall, alongside window embrasures in each wall, some now blocked or destroyed.

Despite centuries of wear and ivy coverage obscuring the uppermost level, Newcastle Castle retains fascinating architectural details that speak to both its defensive origins and domestic evolution. A fragment of bawn wall extending north from the tower hints at a once-larger defensive complex, though recent archaeological testing in 2007 yielded no additional material evidence. The castle’s succession of owners and architectural modifications, from murder holes to Georgian dwelling houses, tells a broader story of how Irish fortified homes adapted from medieval strongholds to more comfortable residences as the centuries progressed.