This National Monument, protected since 1976, represents a fascinating example of multi-period construction that may stretch back to pre-Norman times. The castle’s distinctive curtain wall, built with enormous cyclopean masonry blocks and featuring unusual shouldered window embrasures, suggests origins far older than its medieval tower. Rather than forming a complete defensive circuit, this massive wall, measuring 2.4 metres thick and 4.6 metres high, appears to have functioned as a barrier cutting off access to Knockadoon, similar to a promontory fort, with the waters of Lough Gur providing natural defence on the remaining sides.

The most remarkable feature of the site is its stone causeway, which once provided the only access to the castle across what was then open water. Thomas Dineley’s 1680 drawing shows this impressive structure defended by two gatehouses; one at each end of the causeway, with drawbridges controlling passage. The causeway itself measured approximately 132 metres long and 6.7 metres wide, built solidly of stone to withstand the waters of the lake. Archaeological evidence reveals a deep defensive ditch cutting across the causeway midway along its length, whilst the northern gatehouse, which still partially survives projecting from the curtain wall, features gun loops providing flanking fire and shows evidence of late medieval punch-dressed stonework.

The rectangular tower known as the Black Castle was added to the southwestern angle of the earlier curtain wall, creating the multi-period complex visible today. Though much reduced from its original height of around 10.6 metres, the tower’s remains show it measured approximately 7.6 by 4 metres internally, with walls 2.3 metres thick. Inside the curtain wall, traces of a cobbled roadway lead from the gatehouse entrance, following the base of Knockadoon towards Bourchier’s Castle, located 550 metres to the north. The site’s strategic importance is evident from its commanding position over Lough Gur, controlling access to Knockadoon and the broader landscape of ecclesiastical sites including Killalough Church, just 160 metres to the south.