This citadel stands at the confluence of the Spree and Havel rivers in northwestern Berlin. The Hohenzollern electors of Brandenburg used it as a summer residence, and, in time, it became the chief military center of Prussia. The citadel has been besieged by everybody from the French to the Prussians. The Juliusturm (Julius Tower) and the Palas, constructed in the 13th to the 15th century, are the oldest buildings in Berlin still standing. The main building, accessible by footbridge, houses a local-history museum.