A bastion is a projecting angular element built into a wall or rampart, designed to eliminate blind spots and allow defenders to fire along the face of the fortification rather than simply outward. The design arrived in Britain and Ireland from continental Europe during the sixteenth century, and whatever was begun here after 1552 appears to have been among the first applications of that thinking on Irish soil.

The site has accumulated layers of military purpose across four centuries, each generation leaving something behind. Lythe's map of 1571 shows it already under construction, an unfinished square fort completed on three sides with three bastions and a gun battery at the shoreline. By around 1625 it appears on another map labelled, puzzlingly, 'King John his fort', a name whose origin remains unexplained. A 1685 map by Phillips records not only this fort but a second structure a short distance to the south, a castle with earthen outworks known as Prince Rupert's Tower, which is thought to have originated during the mid-seventeenth-century civil war years. Both of these earlier works were superseded around 1798 by the construction of Carlisle Fort proper, a diamond-shaped enclosure with a rock-cut moat on the landward side and detached gun emplacements following the contours of the slope down toward the sea. Behind the main bastion there was once a barrack for several officers and 155 artillery men; a report from 1804 lists 53 pieces of ordnance on site, though by 1822 the garrison had dwindled to a master gunner and six men. Around 1870, a wide zig-zag ditch was dug to enclose all the existing works, fitted with caponiers, covered passages within the ditch that allowed defenders to fire along it. New barrack buildings and a pier were added at this time, and some of that construction involved an unusually early use of mass concrete. The fort was handed over to the Irish government in 1938, renamed Dúnan Dáibhisigh, and remains in use today as an army training camp, which means most of the surviving structures, including the barrack buildings, are still standing on active military land rather than presented as a heritage site.