Because Hesse-Darmstadt was a member of Napoleon's Confederation of the Rhine, it was forced to give up a considerable amount of territory at the 1815 Congress of Vienna. The territory of the Duchy of Westphalia, which Hesse-Darmstadt received from the 1803 "Reichsdeputationshauptschluss", was given to the Kingdom of Prussia. However, it received some territory on the western bank of the Rhine, including the important federal fortress at Mainz. 