Alcatraz Island's mid-19th-century fortifications

Alcatraz Island was fortified in the 1850s with gun batteries and served as part of San Francisco's harbor defenses; it was garrisoned and used to detain Confederate sympathizers and military prisoners during the Civil War. The island's military role continued through the 19th and early 20th centuries before it became a federal penitentiary in 1934. Visitors reach the historic fortifications and prison ruins via ferry service departing Pier 33.