Castillo de San Marcos National Monument , location of oldest Masonry Fortress the United States , built by the Spanish in Matanzas Bay between 1672 and 1695 to protect the city St. Augustine in the northeast Florida . It was established as Fort Marion National Monument in 1924 and renamed in 1942. The park has an area of about 10 hectares.
The fort is a bastioned structure of Coquina (shell stone) with walls 10 m high and 4 m thick, surrounded by a moat (now dry). It was the 10th fort built on the site, with the previous structures having been built of wood. It played an important role in the struggle between the Spanish and the British for control of the south-east (ca. 1670–1763) and was later held by the British (1763–83). . After the US took over Florida (1819–21), the fort's name was named in honor of the American colonial soldier Francis Marion Changed from Castillo de San Marcos to Fort Marion . It later served primarily as a military prison. During the second Seminole War Indians were held there (1835–42) and in the 1870s and 1980s, Confederate soldiers were American Civil War inside imprisoned . The fort was decommissioned in 1900, having never been attacked or conquered, only changing hands through agreements and treaties.