Complex sentence: 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.
Split into simpler sentences: At the Congress of Vienna, the Grand Duke was forced to cede Westphalia to Prussia. In exchange for this he received a piece of territory on the left bank of the Rhine, including the important federal fortress at Mainz.