St Augustine - Castillo de San Marcos National Monument

Spanish forces built Castillo de San Marcos overlooking Matazanas Bay between 1672 and 1695 to protect their treasure fleets from pirates and an aggressive international armada. It has 12-foot (3.64m) thick, 33-foot (2.5m) high walls made of coquina, a local shellrock. It is reputedly the oldest masonry fort in the U.S. Three sides of the fort are surrounded by a moat and the fourth contains a drawbridge. There are triangular bastions on each of the four corners and the gun deck once held around 75 canons.

Castillo de San Marcos has changed hands a number of times throughout its history, from the Spanish to the English and back again and then finally to the Americans when the territory was sold to them in 1821.

The Americans used it during the Seminole wars of the 1830's, mostly as a prison. Court-martialed American soldiers were also imprisoned here until the late 19th century. It was decommissioned in 1900 and made into a national monument in 1924.