The Spanish Fortress, also known as Castillo de San Marcos, is a historic military structure located in St. Augustine, Florida. Built by the Spanish between 1672 and 1695, it was designed to protect the city from invasions and attacks. The fortress is made of coquina, a unique shellstone that provides durability against cannon fire and erosion. Today, the Spanish Fortress is a National Monument and a popular tourist attraction. Visitors can explore its well-preserved walls, learn about its history, and enjoy stunning views of the surrounding area. The site reflects the colonial history of the United States and the influence of Spanish architecture.