Castillo de San Marcos is a historic Spanish fort located in St. Augustine, Florida, known for being the oldest masonry fort in the continental U.S.

Built between 1672 and 1695, Castillo de San Marcos is an iconic symbol of St. Augustine's Spanish colonial history. Constructed using coquina, a unique shell-based material, the fort proved incredibly durable and resilient against attacks. It played a key role in protecting Spanish interests in the New World and has witnessed centuries of conflict and change. Today, it is a National Monument managed by the National Park Service and attracts visitors for its well-preserved architecture and fascinating history.