Original: Subsequent to World War II the Rathaus served as the city hall of West Berlin until 1991 when the administration of the reunited City of Berlin moved back to the Rotes Rathaus in Mitte.
Simplified: Subsequent to World War II it served as the city hall of West Berlin until 1991 when the administration of the reunited City of Berlin moved back to the Rotes Rathaus in Mitte.