The Inner Harbour exists because of a geological mistake the river made and the people of Duisburg refused to accept. Around the year 1000 the Rhine wandered west, leaving the medieval town landlocked. For four centuries Duisburg sulked, a small agricultural settlement on a dying side-channel, watching the trade routes pass it by. Only in the nineteenth century did engineers dig a new connection from the Rhine, cutting first the Outer Harbour and then extending it eastward into what became the Innenhafen. The timber yards arrived. Then the grain mills. By the high years of the Industrial Revolution the basin was lined with brick warehouses tall enough to throw shadows across the water, and the air smelled permanently of milled flour and Rhine damp.

When the mills closed, the city did not so much abandon the Inner Harbour as turn its back on it. The medieval city walls still ran along the south edge of the basin, and for decades they did exactly the job they had originally been built for: keeping people out. Behind them, warehouses moldered. Cranes rusted. The water grew opaque. To walk in central Duisburg in the 1980s was to never quite realize there was a working waterfront a few hundred meters away. The first cracks in this strange amnesia came at the end of the decade - someone opened a section of the wall, someone built a small housing quarter on Corputiusplatz - and then the IBA Emscher Park arrived in 1989 with the radical idea that the old industrial Ruhr could become something other than ashamed of itself.

Foster's master plan, delivered in 1994, did one essential thing: it gave the water back to the city. The walls came down at the edge. New apartment blocks and office buildings curve along the basin, including the Eurogate building whose semi-elliptical face follows the bend of the Holzhafen. The grain warehouses that defined the skyline were kept - of course they were kept. The brick Küppersmühle was scooped out by the Swiss architects Herzog and de Meuron and reborn as one of Germany's most respected contemporary art museums. The Werhahnmühle next door became a children's museum, then a Legoland Discovery Centre. And on the southern bank, where part of the Jewish quarter had stood before 1938, the Israeli artist Dani Karavan laid out the Garden of Remembrance - concrete pathways and broken walls inscribed with words, designed so that you walk through the memory rather than past it.

Inside the Garden of Remembrance, the architect Zvi Hecker built a synagogue for the Jewish community of Duisburg-Mülheim-Oberhausen, its angular brick walls splayed like the pages of an opened book. A pedestrian bridge called the Buckelbrücke - the buckle bridge, because that is what it looks like - arches over the water to a marina that did not exist twenty years ago. More than fifteen restaurants and bars have moved into the old warehouse arcades. The Cultural and City History Museum holds the surviving Mercator globes, made by the cartographer in this very town in the sixteenth century. None of this looks like a developer's pastiche of industrial heritage. The brick is real brick. The cranes that remain are real cranes. Foster's intervention was to add water access, not nostalgia.

What the Innenhafen demonstrates, more usefully than almost any other example in Europe, is that you do not need to demolish to renew. Almost every important building here is repurposed: a grain mill into a museum, a warehouse into the Land Archive of North Rhine-Westphalia, the old Schwanentorbrücke moving bridge into the photogenic backdrop of a thousand restaurant terraces. On a still evening at the blue hour, when the lights come on along the Marina and the office towers double themselves in the water, it is easy to forget that this district was a no-go zone within living memory. The basin still has a slightly industrial smell when the wind comes from the right direction. The flour is long gone, but the bones are honest.