The star-shaped fortification on the eastern shore of Kinsale Harbour was designed to bristle with defences facing the sea, yet the land approach behind it sits on higher ground, leaving the whole structure overlooked from the rear. That particular weakness was not merely theoretical. It was exploited with decisive effect within a decade of the fort's completion.

Built between 1678 and 1683 on the site of an earlier structure called Ringcurran Castle, the fort was designed by architect William Robinson and covers roughly ten acres. In plan it forms an irregular polygon, with three bastions, named Cockpit, Flagstaff, and North, angled toward the landward side, and two demi-bastions along the shoreline. The ramparts linking them rise to around fourteen metres, and the seaward defences are arranged in tiers: an outer line of gun batteries along the shore, and a third tier formed by Orrery's battery to the south. A bastion in early modern fortification design is an angled projection built into the wall of a fort so that defenders can fire along the face of the adjacent walls, eliminating blind spots. Here, despite the sophistication of that geometry, Williamite forces besieging the fort in 1690 simply occupied the higher ground to the rear and forced the garrison to surrender after thirteen days. The original entrance gateway was destroyed in that same siege; the one visible today between the Flagstaff and North bastions is an 18th-century replacement.

The exterior walls and bastions have remained broadly unchanged since the 17th century, but the interior tells a more layered story. Throughout the 18th and 19th centuries the fort served as an active military barracks, and officers' quarters, soldiers' quarters, a governor's house, and a guardhouse were all inserted within the old ramparts. A 17th-century vaulted magazine and its associated diagonal blast wall, a feature designed to deflect the force of an accidental explosion away from the rest of the fort, still survive alongside sections of the citadel's inward-facing ramparts. The fort is open to the public and administered as a National Monument.