When the tide goes out of the Wadden Sea, the seabed between the mainland and Neuwerk becomes walkable — a wet, ribbed plain of mud and sand crossed by a line of poles driven in to mark the safe route. People walk it. Others ride a Wattwagen, a horse-drawn mudflat coach that sets out from Sahlenburg or Duhnen on the Cuxhaven shore and takes around two and a half hours to cross. The tide decides the timetable, so departure times shift day by day, and the margin for error is not generous. Standing at intervals along the route are elevated cages on stilts: rescue beacons, meant for anyone the flood catches too far out. Climb in, wait for the water to fall, or fire the flare and summon a boat. The flare works. It also comes with a bill and a fine — one way of making sure people read the tide tables first.

Hamburg's interest in this patch of mud was never scenic. The Elbe was the city's artery, and a Hanseatic port that could not guarantee safe approach was a port in trouble. On 24 April 1286, Hamburg's merchants, together with those of Bremen and Stade, secured a deed from Albert II, Duke of Saxony, and his young nephews Albert III, Eric I and John II — the co-ruling lords of the Land of Hadeln, which then included this island — permitting them to maintain a permanent fire on a mudflat island called O, or Nige O. On 1 November 1299 Albert III and John II went further and allowed the merchants to build a fortified tower. They called it the new work: Neuwerk. The 35-metre tower was finished in 1310, and an alderman and ten men-at-arms promptly moved in. The oldest surviving document that names the island is a Frisian contract of 1316, which still calls it Nige O. Accounts differ over how much of the present stonework is original and how much comes from a later fourteenth-century rebuilding, with 1367, 1369 and 1377 all cited. Either way, the tower is Hamburg's oldest existing building and the last surviving piece of the city's fortifications.

For its first three centuries the tower was a daymark and a garrison, a shape on the horizon rather than a light. A beacon fire began burning there at night in 1648. Full conversion to a lighthouse came in 1814. None of it made the approach safe. Ships kept coming to grief on the flats, and Neuwerk kept receiving what the sea gave back — which is why the island has a graveyard consecrated in 1319 and known as the Friedhof der Namenlosen, the graveyard of the nameless, where bodies washed ashore were buried without names to put on them. It was redesigned in 1900. These days the dead are taken to the mainland. The tower's own worst moment came in the First World War, when a shell destroyed the beacon and its signalling gear. On 3 September 1915, lightning struck the Zeppelin LZ 40 — the navy called her L 10 — and she went into the North Sea near Neuwerk. All twenty men aboard died.

Dikes ring the island, and you can walk the circuit in an hour. Outside them lies the Outland, salt marsh that floods and drains and serves as a nursery for oystercatchers, common terns, sandwich terns, black-headed gulls and herring gulls. Farmers pasture cattle and horses on the northern Outland through the summer. Neuwerk is car-free. Its Hamburg quarter also takes in Scharhörn and Nigehörn — the latter built deliberately out of dredged sand in 1989 — both closed to visitors and given over entirely to birds. Since 9 April 1990 the islands and the flats around them have formed the Hamburgisches Wattenmeer National Park. Visitors can climb the lighthouse to a viewing platform for a small fee; there are seven guest rooms inside the tower itself and a hostel in the building beside it. In summer the MS Flipper crosses from Cuxhaven's Alte Liebe pier at high water, about ninety minutes each way.

Neuwerk's twentieth century was largely a matter of paperwork. The Greater Hamburg Law made it Prussian in 1937, and after the Second World War it landed in the new state of Lower Saxony. In 1969 Hamburg got it back, along with Scharhörn, in exchange for surrendering the city's remaining rights to its harbour estate in Cuxhaven — a swap driven by an ambition that never happened. Hamburg wanted a deepwater port out at Scharhörn, and the plan was eventually abandoned. What survives from the island's era of self-sufficiency is quieter: in 1946 an 18-kilowatt wind turbine fifteen metres in diameter went up to save diesel, and it powered the lighthouse and the island's houses for about twenty years, until a submarine cable from the mainland made it redundant. The North Sea coast would come back to that idea later.