It is the oldest masonry fort and best-preserved example of Spanish colonial fortification in the continental U.S. It served as a haven for both soldiers and citizens during multiple sieges against the British. The fort's location on the West Bank of the Matanzas Bay allowed its guns to protect the harbor entrance as well as ground to the north against a land attack.
We really loved touring this impressive fortification. Don't miss it if you every visit St. Aug.
Link for more info: Castillo de San Marcos - Federal Sitekgh