What makes Spike Island stranger still is how many different purposes that structure has served, and how much older history it quietly buried in the process. The fortification is a star fort, a design in which angled bastions project outward from a central body, eliminating the blind spots that had made medieval walls vulnerable to cannon fire. It is surrounded by a broad dry ditch and an outer glacis, the sloped earthwork that deflects incoming fire, and inside, ranges of barrack buildings are arranged around a large parade ground.

The island's military history did not begin tidily. A battery was erected there in 1779 but abandoned by 1783. Work on a more permanent fortification, designed by the military engineer Charles Vallancey, began in 1791, though the original intention had been for something considerably smaller and simpler. What emerged over the following decades was on an altogether different scale: an extensive artillery barracks was noted by Samuel Lewis in 1837, the barracks themselves had been erected by 1806, and an ordnance depot was established by 1811. As late as the 1840s, observers noted the works were still incomplete, yet the fort had broadly taken its present form by 1842, with construction continuing at least to 1860. Between 1847 and 1883, the fort was repurposed as a convict prison, a function that layered a very different kind of history onto the military fabric. To the west, the remains of what was described as a large and handsome military hospital are still visible, and further buildings for those connected with the establishment once lined the northern shoreline. The fort is today used as a civilian prison, meaning much of it remains inaccessible.

Beneath all of this institutional weight lies something older and almost entirely invisible. The island has been identified with an early ecclesiastical site known as Inispicht, though no surface traces of it survive. A map dating to 1625 appears to show a ruined church on the island, but by the time Vallancey was surveying the ground in the 1790s, it had disappeared from the record entirely. A castle or tower house visible on a 1587 map had likewise vanished. Whatever was once there, the fort consumed it.