Lenan Fort, Lenan, Co. Donegal

This is not a folly or a boundary marker but a deliberate barrier, cut to sever the fort from the mainland and defended in turn by three concrete blockhouses built in 1916, one pentagonal, two four-sided, each reached by a covered way that drops down steps to rifle loopholes shuttered in iron. Above the ditch, a fire trench was dug into the higher ground near the cliff edge, positioned to sweep the north-eastern stretch below. It is a strange sort of architecture, functional and slightly forbidding, built entirely around the logic of denying an enemy a few dozen metres of ground.

The battery itself dates to 1902, part of the coastal defences protecting Lough Swilly, one of three deep-water anchorages, along with Cork Harbour and Berehaven, that Britain kept under its own control after the 1921 Treaty as so-called Treaty Ports. Three gun positions were cut into the cliff facing north, tied together by sunken roads and served by underground magazines and shelters, with a battery commander's post and position-finding cells set upslope. To the south lay the domestic side of the fort: barrack blocks in three and six-bay ranges, a guardhouse cut into the hillside with its own concrete retaining wall, and an octagonal water tower whose cast-iron tank still carries the stamp of its makers, Douglass Bros of Blaydon-on-Tyne, dated 1901. Corrugated-metal huts added around 1914 have mostly collapsed to their concrete foundations and brick chimneystacks, a reminder that the fort's population expanded and contracted with successive wars. Ireland did not take possession of Lenan until 1938, when the Treaty Ports were handed back, and the 6th Coastal Battery of the Artillery Corps moved in soon after. Through the Second World War the fort held a garrison of some 120 men, watching over the lough's approaches, before shrinking to a single caretaker in 1946 and being abandoned altogether in 1952.

What remains today is a layered record of over half a century of military engineering, from Edwardian brick barracks with their red-brick chimneys and cast-iron rainwater goods to First World War-era blockhouses and the utilitarian huts of a garrison at full strength. The buildings show their age unevenly; some barracks retain timber-framed windows and sandstone lintels, while others survive only as partial walls or foundations. The ditch and its blockhouses, though, remain the most legible part of the site, a clear demonstration of how a peninsula could be turned into an island by a few metres of excavation and a line of loopholed concrete.