The Castillo de San Marcos, built 1672-1695, served primarily as an outpost of the Spanish Empire, guarding St. Augustine, the first permanent European settlement in the continental United States, and also protecting the sea route for treasure ships returning to Spain. Although the Castillo 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 US army until 1899. The Castillo was made a National Monument in 1924 and became part of the National Park system in 1933. In 1942, Congress restored the original name. The park consists of the original historic Castillo fortress itself with its attendant grounds, some 25 total acres.