Mainz-Kastel is a district of the city Wiesbaden, which is the capital of the German state Hesse in western Germany. Kastel is the historical bridgehead of Mainz, the capital of the German state Rhineland-Palatinate and is located on the right side of the Rhine river. Kastel faces the historical center of Mainz and the two cities are connected by a road bridge. Kastel is located about one kilometer below the mouth of the river Main, where it flows into the Rhine.