It also protected the sea route for treasure ships returning to Spain. Although the Castillo, built 1672-1695, has served a number of nations throughout its history, it has never been taken by military force.
During the 18th century, the Castillo went from Spanish control to British and back to the Spanish, all by treaty. The Spanish remained in power in Florida until the area was purchased by the United States in 1821. Called Fort Marion at this time, the Castillo was used by the U.S. army until 1899. The Castillo was made a national monument in 1924.