Frederick William IV of Prussia
Peter I of Russia studied the fort in 1697 while touring Europe. The fort was used as a state and military prison until 1825; Ludwig Yorck von Wartenburg was imprisoned there for insubordination from 1780–81. The Friedrichsburg Gate was completed during the reign of King Frederick William IV of Prussia. The fort was remodeled in 1852 during the reign of King Frederick William IV of Prussia, with construction of the stately Friedrichsburg Gate, new walls, and expansion of the citadel with four round towers. By the end of the 19th century, however, the fort was used only for military storage.