The Castillo de San Marcos is a historic fortress located in St. Augustine, Florida. Built by the Spanish between 1672 and 1695, it is the oldest masonry fort in the continental United States. The structure is made of coquina, a unique shellstone that provides durability against cannon fire and erosion. The fort served to protect the Spanish colony from invasions and was later used by the British and the United States. Today, it is a National Monument, attracting visitors who can explore its walls, learn about its history, and enjoy views of the surrounding area.