By 1813 the Napoleonic Wars had been grinding for over a decade, and Britain was simultaneously at war with the United States. The English Channel was a working battleground; the Baltic timber trade, vital for Royal Navy masts and shipbuilding, depended on convoys that took the long way around the north of Scotland. The bay of Longhope, sheltered behind South Walls, was a natural rendezvous point where merchantmen could gather and wait for a Royal Navy escort. It was also exposed to a particular kind of attack. French and American privateers - privately owned ships licensed to take enemy commerce - operated independently, struck fast, and made no attempt to engage warships. A handful of armed sloops could devastate a waiting convoy. The Admiralty wanted Longhope defended. In 1815 two Martello towers and a battery went up: Hackness on the south side of the bay and a twin at Crockness on the north.

The design came from a tower at Mortella Point in Corsica, where in 1794 a Royal Navy squadron had taken heavy damage trying to subdue a small Genoese gun position before eventually capturing it. The British were impressed enough to copy the form - the spelling shifted, the basic idea did not. Thick, sloping walls of solid masonry could absorb cannon fire from ships at sea. A single heavy gun mounted on a turntable on the flat roof could fire in any direction. A small garrison - typically a dozen soldiers - could hold the building against a much larger landing force. Between 1804 and 1815 the British built more than a hundred Martello towers from the south coast of England to Ireland, Canada, the Caribbean, South Africa, and now finally to Orkney - the northernmost of the lot. By the time Hackness was finished, Waterloo had been fought. The wars were over.

Defenses built at the end of a war are usually obsolete on completion. Hackness was no exception - it never saw an enemy in the Napoleonic context it was built for. But the Royal Navy kept finding reasons to maintain it. In 1866, responding to the perceived threat from the American Fenian Brotherhood, the towers and battery were upgraded with new guns and additional structures. The barracks beside Hackness Battery, where a small garrison lived in conditions that would have struck a Caribbean Martello soldier as Arctic, were periodically refurbished. Even in the twentieth century, Scapa Flow's importance as a fleet anchorage kept the wider Hoy defences active through two world wars, though by then the Martello towers themselves had become curiosities rather than working fortifications.

Hackness Martello Tower and Battery is now in the care of Historic Environment Scotland and open as a museum. Visitors climb the narrow internal stair to the gun deck on top, where the original turntable mounting is still visible and the view sweeps across Longhope, Aith Hope, and the dark hills of Hoy. The Crockness twin across the bay is also extant, though less restored - a private property, visible from the road. Together they remain among the cleanest surviving examples of the form: thick walls, narrow embrasures, single heavy gun, designed for one job and built when it was almost too late to do it. The Martello tower is one of those forms - like the lighthouse or the windmill - that survives because its geometry is what its function looks like. Hackness is the northernmost surviving example. There are none beyond it.