But that stretch of masonry, along with the surviving east wall and a squared northeast corner bastion, is what remains of a seventeenth-century military fort, one whose original form is preserved not in any ruin but in an eighteenth-century map hanging inside a cathedral.

The fort was built between 1641 and 1652 by Captain Robert Gookin, a period that places its construction squarely within the upheaval of the Confederate Ireland wars and the subsequent Cromwellian conquest. Bastioned forts of this type were a distinctly European military design, developed to resist artillery: rather than simple curtain walls, they featured angled projections, or bastions, pushed out at the corners so that defenders could fire along the faces of the walls and eliminate blind spots. Gookin's fort was square in plan, with pointed bastions at each corner, and its layout is clearly legible on a 1788 map now displayed in Ross Cathedral, the early medieval foundation that still anchors the small town of Rosscarbery. That a map of a largely vanished military structure should end up on the walls of a cathedral is its own quiet curiosity. What survives on the ground includes the north wall, running some 56 metres east to west, the east wall at roughly 45 metres, and the northeast corner bastion, which is square rather than pointed in its surviving form. The walls stand to an exterior height of around four metres with a thickness of approximately 0.6 metres, and may incorporate fabric from the fort's original construction phase.