Constructed in 1672 by the Spanish forces that landed in St. Augustine, Florida, the Castillo de San Marcos is the oldest masonry fortification in the US. It was captured and occupied by the British troops in 1763, and was renamed as Fort St. Mark for a time. When the US claimed it in 1821, it was renamed after Francis Marion, the revolutionary war hero. It was called in its original name by 1942, when it was restored by order of the Congress.

The Castillo de San Marcos was declared a National Monument in 1924. It is currently open for tours and school field trips. However, make sure to check the weather forecast first before visiting, especially during the months of June through November.