Fort Matanzas sits on a small island about 14 miles south of St. Augustine, accessible by a free National Park Service ferry that runs throughout the day. The coquina watchtower was built by the Spanish between 1740 and 1742 to guard the southern approach to the city after a British fleet had demonstrated the vulnerability of the inlet. The tower was small by fort standards — it held a garrison of just a dozen men — but it successfully deterred attack from the south. When Florida passed to the British in 1763, the garrison was withdrawn and the fort was abandoned.
The ferry crossing takes about five minutes and gives good views of the Matanzas Inlet marshes. On the island, park rangers offer regular talks inside the fort. The surrounding marsh and beach are productive for birdwatching — wading birds, shorebirds, and in spring and autumn large numbers of migrating species work the tidal flats. The site is far less crowded than the Castillo de San Marcos and the natural setting is genuinely beautiful. Budget about an hour and a half for the visit, including ferry waiting time. Free admission.