Florida History: What do you know about Castillo de San Marcos, Florida’s oldest man-made structure? You can’t visit St. Augustine without exploring Florida’s oldest man-made structure, the Castillo de San Marcos. Welcome to Florida Time, our weekly column about Florida history. Readers: In a state that embodies transience, Castillo de San Marcos -- Florida’s oldest man-made structure and America’s oldest fort -- wears its age proudly. Up to a million people a year cross its drawbridge without incident. But over three centuries, raiders have tried, and failed, to take the four-sided fort on Matanzas Bay. Sir Francis Drake torched the city in 1586, but it wasn’t until the British sacked the town again in 1668 that leaders decided their wooden forts were inadequate. Work started four years later and it took two decades to build the structure, mostly with coquina blocks -- compressed mixtures of oyster shells and lime mined from the adjacent Anastasia Island. Florida Time archives: Get caught up on the stories you’ve missed Florida History: What do you know about Castillo de San Marcos, Florida's oldest man-made structure?