Siegen looks compact on a map but covers about 115 square kilometres, and roughly sixty per cent of it is forest. That makes it one of the greenest big cities in Germany - a place where you can ride a bus from the main station and be walking under beeches within twenty minutes. The basin sits at a median 290 metres above sea level, with the Sauerland to the north, the Rothaargebirge to the northwest, and the Westerwald falling away to the southwest. Six administrative districts ring the centre; six rural valleys carry roads and rail lines into the surrounding hills. The population pushed past 100,000 in 1975 when the towns of Huttental and Eiserfeld were merged in, and it has hovered around 105,000 ever since, with nearly 20,000 students of the University of Siegen swelling the streets through term.

The river Sieg gives the city its name - a Celtic word, probably - and iron gives it almost everything else. Mining here goes back to La Tene times, more than two thousand years ago; the surrounding mountains are still pierced with old adits and shaft heads, some now hiking attractions, some still sealed. Throughout the Middle Ages and into the modern era the Siegerland was one of Europe's most prolific iron-producing regions, and the Nassau counts who eventually ruled it grew rich on the trade. By the nineteenth century the trade had blossomed under Prussian administration into a constellation of foundries and metalworks, and Siegen became the central market for the whole industry. Mining has now largely ceased, but the inheritance is everywhere: in the dark slag underfoot in some woods, in the engineering tradition the local university teaches, in the names of districts that were once distinct iron towns.

For most of its history Siegen was a Nassau city, ruled from the Obere Schloss on the Siegberg by Counts of Nassau-Siegen and their related branches. One of those branches turned out to be world-historical: through marriage and inheritance the Nassau line acquired the French principality of Orange in the sixteenth century, and produced a son - William, born just down the valley at Dillenburg in 1533 - who would lead the Dutch revolt against Spain and found the modern Netherlands. The town today sits on the German-Dutch holiday route called the Oranier-Route, threading places associated with the House of Orange-Nassau. Other Nassau-Siegen sons did equally remarkable things. John Maurice of Nassau-Siegen governed Dutch Brazil in the seventeenth century and shipped tropical natural history back to Europe; his portrait hangs in the Obere Schloss alongside the Rubens collection.

Rubens's family stayed in Siegen only briefly. The boy was raised mainly in Cologne and then in Antwerp, where he became the most sought-after painter in northern Europe, but his birthplace has not forgotten him: nine of his works hang in the Rubens Hall of the Siegerlandmuseum, including an oil sketch for the Ecce Homo and one of his earliest treatments of the Descent from the Cross. The city that birthed him very nearly did not survive the twentieth century. During the Second World War Siegen was bombed repeatedly because of its critical railway junction - 16 December 1944 saw a particularly catastrophic raid - and on 1 April 1945 the US 8th Infantry Division began the ground assault that finally took the city after three days of fighting. About eighty per cent of the historic core was destroyed. The two stately homes, the Oberes Schloss and the Unteres Schloss, both survived. Postwar Siegen rebuilt around them, with the Nikolaikirche's distinctive crown tower still standing over the Old Town and the city's medieval bones still visible if you look.