The Castillo de San Marcos site is the oldest masonry fort in the United States. It is located in the city of St. Augustine, Florida. Construction was begun in 1672 by the Spanish when Florida was a Spanish territory. During the twenty year period of British possession from 1763 until 1784, the fort was renamed Fort St. Mark, and after Florida became a U.S. territory in 1821 the fort was again renamed Fort Marion, in honor of revolutionary war hero Francis Marion. In 1942 the original name, Castillo de San Marcos, was restored by Congress. The site was never taken by force despite multiple periods of attack.
In 1924, the fort was designated a National Monument. In 1933 it was transferred to the National Park Service from the War Department. In 1942, in honor of its Spanish heritage, Congress authorized renaming the fort as Castillo de San Marcos. As an historic property of the National Park Service, the National Monument was listed on the National Register of Historic Places (NRHP) on October 15, 1966. The National Park Service manages the Castillo together with Fort Matanzas National Monument.