Fort Delaware State Park occupies Pea Patch Island in the Delaware River, where a fortification proposed in the 1790s evolved from an early wooden star fort into the present brick and concrete fort substantially completed by 1860. During the Civil War it served as a Union prison camp for Confederate captives, including many from Gettysburg, and after later use in coastal defense through the Spanish-American War, World War I, and World War II, it became obsolete and was abandoned. Delaware acquired the property in 1947 and opened the park in 1951. Today it preserves both a major military site and important wetlands habitat with a large rookery.