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 was designed to protect the city from invasions and has withstood numerous attacks over the centuries. Constructed from coquina, a unique shellstone, the fort features thick walls and a star-shaped design. Today, the Castillo de San Marcos is a National Monument, attracting visitors who can explore its grounds, learn about its history, and enjoy stunning views of the surrounding area.