The fort is a bastioned structure of coquina (shell stone) with walls 33 feet (10 metres) high and 12 feet (4 metres) thick, surrounded by a moat (now dry). It was the 10th fort built on the site, 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 Southeast (c. 1670–1763) and was later held by the British (1763–83). After the United States acquired Florida (1819–21), the fort's name was changed from Castillo de San Marcos to Fort Marion in honour of the colonial American soldier Francis Marion (Marion, Francis). It subsequently served mainly as a military prison. Native Americans were held there during the Second Seminole War (Seminole Wars) (1835–42) and in the 1870s and '80s, and Confederate soldiers were imprisoned in it during the American Civil War. The fort was decommissioned in 1900, having never been attacked or captured but rather having changed hands only through agreements and treaties.