Köln is at the center of Germany's biggest metropolitan hub: the Rhine-Ruhr. Here, 18 million people live, making it one of the biggest urban areas in Europe. The city and its metropolis sit on the banks of the Rhine, possibly Europe's most historically significant river. This river was the northern edge of the great Roman-Empire, who founded Köln and many of the big European cities during their reign. Throughout history, Köln has maintained itself as one of the most important harbours along the Rhine, and in the past hundred years has developed into Germany's biggest metropolis. Again, like many other centers in Germany, there is an incredible story of reconstruction after 1945. The bridges and most of the buildings you see in these pictures were destroyed in the bombing raids of WWII.