A ferry boat takes you from the mainland over to this National Monument island. I have been to the Castillo De San Marcos in St. Augustine several times throughout my life and every time it is crawling with people.
Fort Matanzas if off the beaten path, leaving you elbow room to explore... even during Florida's high tourist season. The fort, located on Rattlesnake Island, is made of coquina stone, same as the Castillo, placed on top of pine pilings driven into the marshy ground to give stability. There is an educational video at the office before you cross on the ferry, as well as a tour guide who can answer any questions that weren't covered in the video.