CASTILLO DE SAN MARCOS—
The oldest and largest stone fort in the continental United States, built by the Spanish during 1672-1695 with blocks of coquina. The fort endures as the nation’s oldest and only remaining 17th-century stone fort. Open daily from 8:45 am to 5 pm. $7/person ages 16 and up. www.nps.gov/casa. 1 S. Castillo Drive, 904.829.6506.