St. Augustine's most historically significant structure is the Castillo de San Marcos.
This fort was constructed by the Spanish between 1672 and 1695.The need to fortify the tiny garrison town was understood as early as 1586, when it was attacked by Sir Francis Drake, an English corsair whose fleet of twenty ships and two thousand men sacked and burned the town.Later, in 1668, the English pirate Robert Searles assaulted and plundered the settlement. At the same time the threat of English colonization in the north had significantly increased, and Queen Mariana of Spain authorized the construction of a stone fortification.The fort was built of coquina, a type of shell stone indigenous to the area and quarried from Anastasia Island.The Castillo has never been conquered despite attacks by English General James Oglethorpe who attempted to subdue the town in 1740. Firing from the tip of Anastasia Island, he found his cannonballs were no match for the unusual consistency of coquina which absorbed the blast rather than crumbling.
Shortly after Florida became a territory of the United States, the fort was renamed Fort Marion in honor of Revolutionary War General Francis Marion. It retained that name from 1825 to 1942, when the Castillo appellation was restored.
In the 1870s and 1880s, the fort housed Indians detained by the U.S. Government. The Castillo and Fort Matanzas have been under the auspices of the National Park Service since 1933.
The fort is open to the public daily, except for Christmas.