Construction began in 1464 under the city architect Hinrich Helmstede, paid for by a legacy of 4,000 Lübeck marks from a councillor named John Broling, and finished in 1478. The design is two faces on one building. From inside the city the round towers and the central block read as a single generously windowed wall. From the field side - the side an enemy would see - the towers bulge three and a half metres beyond the block, pierced by only a few small openings, embrasures and gun chambers. Wall thickness tells the same story: three and a half metres facing the field, under a metre facing the city. The asymmetry is grim and practical: if the gate ever fell, Lübeck wanted to knock it down quickly from behind rather than leave an enemy a bulwark. The windows above the passage were designed for pouring pitch or boiling water on whoever stood below.

What survives is the middle one. Lübeck's western exit was defended in depth, and over five centuries there were four Holsten Gates in a row. The innermost guarded the Trave bank beside the Holsten Bridge, first mentioned in a deed signed by the king of Denmark in 1216, renewed in 1376, downgraded to a half-timbered gate in the seventeenth century, replaced by iron in 1794 and pulled down in 1828. The Outer Holsten Gate, the Renaissance Gate, went up in 1585 - probably by the city architect Hermann von Rode, working from Dutch models - and stood only twenty metres beyond its predecessor, blocking the view. A fourth followed in 1621, built into new ramparts under the engineer Johann von Brüssel and topped with an octagonal tower. It was the last built and the first to go, in 1808. The names shifted every time one appeared or vanished, which is why the record is a muddle.

By the mid-nineteenth century the gate was a nuisance: in the way of the railway, visibly tilting, and leaning enough that engineers feared the altered loads would bring it down. In 1855 a petition with 683 signatures asked the Senate to finish it off. But sentiment was turning. Three years earlier August Reichensperger had written that even Lübeck, once the proud head of the Hanseatic League, could not endure the reflection of its own former glory - that it maimed and cropped and covered up so assiduously that modern enlightenment would soon have nothing left to be ashamed of. King Frederick William IV of Prussia dispatched his curator of monuments, Ferdinand von Quast, to salvage what he could. The argument dragged on until 1863, when Lübeck's citizens voted to restore rather than demolish - by a majority of one. Work ran to 1871. The settling was only halted in 1933-34, with reinforced concrete anchors and iron rings around the towers.

Both inscriptions on the gate are younger than they look. Over the city-side passage sits SPQL - Senatus populusque Lubecensis, a straight lift from Rome's SPQR - flanked by the dates 1477 and 1871. The first is wrong; the gate was completed in 1478. The second is honest: that is when the lettering was added, along with the founding of the German Reich it was meant to celebrate. Before then there had been nothing there - and nothing would have been visible anyway behind the high walls that once masked the gate's lower storeys from inside the city. On the field side runs concordia domi foris pax - harmony within, peace outside - a compression of a longer motto carried by the demolished outer gate. Two cast-iron lions of 1823, attributed to Christian Daniel Rauch, keep watch outside. One sleeps; the other stays awake and watches the sleeper.

A gate nobody wanted in 1855 became national shorthand. The marzipan house Niederegger put it in its trademark in 1901; the German Association of Cities adopted it as an emblem in the mid-1920s. It appeared on the four highest values of the first postwar Deutsche Mark stamp series in 1948, on the fifty-mark note from 1960 to 1991, and on a two-euro coin in 2006 - and it sits on the horn button of the Amphicar, the amphibious car built in Lübeck in the 1960s, where drivers routinely mistake it for a castle. The interior has been a museum since 1950, when it acquired a torture chamber the gate had never contained; that went in the 2002 redesign, and nine rooms now cover Hanseatic trade, city law and medieval Lübeck. The 2005-06 restoration cost around a million euros. Days after the scaffolding went up, someone cut out the swastika left from 1934 - reputedly the last still in place on a public building in Germany - and carried it off. A plate dated 2006 fills the hole.