Alcatraz Citadel, also known as Fort Alcatraz, was the original military defense and prison on Alcatraz Island, off the coast of San Francisco, California, USA. The citadel was built in 1859 as a U.S. Army military defense, and began function as a war camp in 1861 and long-term military prison in 1868. During the American Civil War, the citadel and its batteries provided an important line of defense.