 San Marcos de Apalache Historic State Park interprets the site of the old fort.Today's St. Marks evidently has its roots in American commercial activity that took hold beneath the walls of the fort upon acquisition of Spanish Florida by the U.S. in 1821—before the settlement moved slightly up the St. Marks River to the present position.Various articles in publications like "Florida Historical Quarterly" relate how the fort site later held a government "naval" hospital to meet yellow fever emergencies in the merchant marine.