What the cartographers quietly noted was the ghost of a Jacobite bastioned fort, a layered military site that began as a much older Anglo-Norman motte and bailey, a type of fortification consisting of a raised earthen mound beside an enclosed courtyard, and was reused across centuries of Irish conflict. The fort occupied a narrow ridge of high ground that was, at the time of its last siege, surrounded on three sides by Lough Sewdy, sometimes recorded as Lough Sunderlin, with bog covering the western approach. It was, in short, a natural island fortress that several generations of soldiers found too useful to ignore.

The fortifications were first constructed in the 1640s and then substantially improved by the Jacobite forces during the War of the Two Kings, the conflict between the supporters of the deposed James II and the armies of William III. By the spring of 1691 a garrison of around a thousand men with two cannon, detached from Athlone, held the place. The defences were formidable: a double ditch backed by a stone wall, a hornwork (an outwork shaped roughly like a bull's horns, projecting forward to protect the main entrance), a bastion, and a palisaded ringfort at the peninsula's tip. George Story, a chaplain travelling with the Williamite army, described the site in careful detail, noting the ditch around the old earthen fort as twenty feet broad and ten feet deep. He also recorded the more domestic history of the place: a widow named White had sheltered her flock of sheep and cattle on the peninsula the previous year when Lieutenant-General Douglas had passed through. The Williamites had bypassed the fort then, judging it too poor to garrison; the Jacobites saw it differently. Story recorded the siege almost with the detachment of a spectator, noting that 'this siege was very delightful to our whole Army, who had a view of it from the adjoining hill.' Four batteries were raised by the night of the 7th of June 1691, including one of six guns towards the lough side and another trained on the works near the church. A white flag appeared on the 8th of June, though the Williamite general declined to acknowledge it; breaches were made in the outer and inner works, boats were launched onto the lough, and by eight o'clock that morning the garrison surrendered. Colonel Earl led eight hundred musketeers in through the breach. Within days the Williamites had begun improving what they had just destroyed, adding a line of communication between the outworks and the old earthen fort, a spur bastion, a half-moon, a hornwork, and a platform for eight guns on top of the mound.

What survives on the ridge today includes the remnants of the seventeenth-century earthworks at its southern end, where the drawbridge entrance and the most southerly bastions remain identifiable. A small spur bastion on the western side and what are likely the remains of the most easterly internal bastion have also been recorded on the ground. Whether these earthworks reflect the Jacobite defences, the subsequent Williamite improvements, or some combination of both is not entirely clear. The old motte still dominates the northern end of the ridge, as it has since the Anglo-Norman period, while the route the Williamite army would have taken from the village to the fort entrance ran along the eastern side of the nearby church, a path that still orients the landscape around the memory of those few June days.