Castle Clinton National Monument

Built between 1808 and 1811 as West Battery (later Castle Clinton), the masonry fort at the southern tip of Manhattan served military, entertainment, and immigration functions over the 19th century and briefly operated as an immigration depot before Ellis Island opened. The circular granite-and-concrete structure later housed an aquarium and other civic uses; it is administered by the National Park Service as Castle Clinton National Monument. The site is an early example of coastal fortification and urban reuse along New York Harbor.