Pampus is an island the Dutch invented. The water is shallow there, never deep, but it was open water until the late 19th century. Building a fort on it meant building the island first: pile after pile driven into the soft Zuiderzee floor, sand barged in by the thousands of tonnes, the whole platform shaped into an oval ringed by a glacis and a dry moat eight metres wide. The fort sits low to the waterline, three storeys of bricks and concrete with most of its bulk dug into the made earth. The Dutch government spent 800,000 guilders on it, roughly eight and a half million euros in today's money - cheap, by the standards of fortifications, for a piece of country that did not previously exist.

Commissioned in 1895, Pampus was one of only four forts in the entire Defence Line of Amsterdam fitted with heavy artillery. Two hydraulically operated cupolas on its roof held two Krupp 24-centimetre guns each, lifted into firing position by electric hoists from the magazines below. Each gun fired a 280-kilogram shell up to eight kilometres. Each crew - an NCO and six gunners - could manage one shot every six minutes. Around the moat, four Gardner machine guns from 1890 sat on garrison mounts. Two smaller quick-firing 57-millimetre cannons covered the close approaches. The fort had quarters for 200 men. It only ever held that many during the First World War, and in that war, as in every other, it stood watch over an enemy that never came.

Pampus was a single piece of an enormous idea: the Stelling van Amsterdam, a 135-kilometre ring of 42 forts encircling the capital, designed to be defended by flooding the polders inside the ring waist-deep, just deep enough that an army could neither wade nor row through. UNESCO recognised the entire Defence Line as a World Heritage Site in 1996. The system was obsolete almost before it was finished. Aircraft and long-range artillery rendered shallow inundation useless, and the 1932 Afsluitdijk cut the IJsselmeer off from the open sea anyway, leaving Pampus guarding a lake. The military gave it up on 15 July 1933 and walked off the island, leaving the guns behind.

The Second World War found uses for the abandoned fort, all of them grim. Between 1941 and 1943 German occupation forces used Pampus as a target range, lobbing 250-kilogram practice bombs - hollow concrete shells filled with chemical-loaded glass tubes that burst on impact and marked the hit with coloured smoke. They eventually stripped out the Krupp guns and the cupolas for scrap. In the brutal Hongerwinter of 1944, when Amsterdammers were starving and freezing, people walked across the frozen IJmeer to Pampus and tore the timber out of the fort to burn for heat. After the German surrender, the island became a place to dispose of unexploded ordnance, ammunition that could no longer be safely defused was simply detonated where it lay.

Pampus today is owned and maintained by the Stichting Pampus, which bought it in 1990 and has lived in it continuously since 1992. A 2007 restoration opened most of the fort to visitors, and from April to October a small ferry runs from Muiden, twenty minutes across the water. There is one gun on the island again. It is an 88-millimetre piece from a German minesweeper wrecked off Terschelling in 1917, dredged up decades later and donated to the foundation in 2003. It points out across the IJmeer at nothing in particular, which is exactly what Pampus has always done.