Oldest remaining military structure: Barracks of Fort Wayne (c. 1848)
Originally the location of burial sites for Indigineous tribes, this fortress was constructed due to tensions with British Canada in the 1830s. It was also used during the American Civil War, the Spanish-American War, World War I, World War II, the Cold War, and the Vietnam War, when the Army discontinued its use. While it has been largely vacant in recent years, its now poised for a massive redevelopment that will include a number of organizations and businesses, thanks to federal funds.
Photo is public domain, Wikimedia Creative Commons