The Castillo de San Marcos is considered by many to be the most impressive of all the historical sites in St. Augustine. Many thousands of people come each year to view the imposing fortress and to learn about the fascinating years that it served the city. Constructed of coquina stone that was shipped in from Anastasia Island, the Castillo is the oldest remaining European fort in the United States. It was built to protect and defend the city of St. Augustine which belonged to the Spanish at the time. Later, the structure was used for various reasons including holding Indian prisoners during the Seminole Wars and Confederate soldiers during the Civil War. A tour of the bastion allows visitors to see the walls that withstood many battles of colonial warfare and to witness the ways in which it protected the city. Cannon firings, historical re-enactments and other events are also a possibility—these activities are held throughout the year.