At the Congress of Vienna, the Grand Duke was forced to cede Westphalia to Prussia, in exchange for which he received a piece of territory on the left bank of the Rhine, including the important federal fortress at Mainz.
The Grand Duke was forced to cede Westphalia to Prussia. This was at the Congress of Vienna. This was in exchange for which he received a piece of territory on the left bank of the Rhine. This was including the important federal fortress at Mainz.