During the Middle Ages, much of the region that was later called the "Ruhrgebiet" was situated in the County of Mark, the Duchies of Cleves and Berg and the territories of the bishop of Münster and the archbishop of Cologne. The region included some villages and castles, and was mainly agrarian: its loess soil made it one of the richer parts of western Germany. The free imperial city of Dortmund was the trading and cultural centre, lying on the Hellweg, an important east–west trading route, that also brought prosperity to the town of Duisburg. Both towns were members of the Hanseatic League.