Castillo de San Marcos National Monument

The Castillo de San Marcos monument preserves the oldest masonry fort in the continental United States, begun by the Spanish in 1672 to protect their Saint Augustine settlement, established in 1565. Erected over the course of 13 years, the massive structure, with walls 13 feet thick at the base and almost 30 feet high, was built of local coquina limestone and surrounded by a moat. Its defenders successfully withstood and repulsed two sieges, in 1702 and 1740. When the Florida territory was ceded to the United States by Spain in 1821, the fort's name was changed to Fort Marion.