Castillo de San Marcos Dog Policy
Castillo de San Marcos National Monument is Somewhat Dog Friendly
Castillo de San Marcos was built by the Spanish to defend Florida, and is the oldest masonry fort in the continental U.S.
Castillo de San Marcos National Monument is a somewhat dog friendly destination. Dogs are allowed outside the fort on a leash, but not inside, so you'll miss the main attraction if you bring your dog.
They run a virtual BARK Ranger program that you can read more about at https://www.nps.gov/articles/castillo-de-san-marcos-bark-ranger.htm