The castle’s strategic position once made it nearly impregnable; originally surrounded by Lough Swilly on three sides and protected by bogland to the north, though much of this natural defence has since been reclaimed. First documented in a 1587 land grant to Sir John O’Doherty, the castle reached its zenith around 1601 when it boasted formidable defences including a thick rampart, two casemates, and five pieces of artillery. The fortification changed hands several times during the turbulent early 17th century, serving as Cahir O’Doherty’s stronghold until his 1608 revolt, after which it passed to the Chichester family who added substantial buildings within and beyond the original bawn.

The keep itself showcases sophisticated military engineering typical of its era, constructed from rough boulders and rubble bound with coarse sea sand mortar. Rising three storeys high with circular towers at the northeast and southwest corners, the structure bristles with numerous gun loops, reflecting the importance of firearms in late medieval warfare. The southwest tower houses a spiral staircase, whilst the interior layout reveals a complex arrangement of vaulted chambers, each serving specific defensive and domestic purposes. Particularly noteworthy are the various vaulting techniques employed throughout; barrel vaults on the ground floor, slightly pointed vaults in the side chambers, and corbel vaulted tower rooms, some still bearing traces of the original wicker centering used during construction.

Archaeological evidence suggests the entrance was likely positioned in the south wall rather than the east as depicted in period illustrations, flanked by a small chamber and the stairwell for maximum defensive advantage. The building’s windows tell their own story of architectural evolution; most have lost their dressings over time, save for one remarkable survivor on the second floor’s north wall, which retains its cut stone dressings with a blunt pointed head and quarter round moulding. By the 19th century, the castle had fallen into decay; it was already roofless by 1833, suffered further damage when many windows were destroyed around 1890, and lost a corner turret to a storm in 1825. Today, whilst the massive external fosse that once provided additional defence has been almost entirely ploughed out, the keep’s substantial walls continue to dominate the hilltop, offering visitors a tangible connection to Donegal’s contested past.