As you drive south on Anastasia Island, you head toward what was, in the 1700s, St. Augustine's farthest reaches. With Castillo de San Marcos guarding the town, in 1740 the Spanish created the relatively small Ft. Matanzas to defend their southern flank. A short ferry ride across the Matanzas River takes you to this national monument. Although it's only 15 miles from town, the fort feels eerily remote, and it takes little imagination to picture the demanding lives of the soldiers sent here to protect the young colony. Ferry shuttles are free and run every hour from 9:30 to 4:30, but you need to pick up a boarding pass from the visitor center.