Construction of the Castillo de San Marcos began in 1672 and was completed in 1695. The Castillo de San Marcos in St. Augustine, Florida, is the oldest stone fort in the continental United States. Originally constructed by the Spanish in the late 17th century, the stone Castillo de San Marcos replaced a previous wooden fortification. An imposing Fort, located on the waterway, Castillo de San Marcos … The fortress draws some 650,000 visitors a year, all eager to explore one of the oldest surviving structures in North America. It is a fort built of stone and is considered the oldest masonry fort in the continental United States. The Castillo de San Marcos National Monument is shaped into a masonry star fort made of native, coquina stone. The Castillo de San Marcos is considered a “walk-in” park, open to the public for visits. Today, Castillo de San Marcos serves as a historical monument, a living museum, a somber warning against the atrocities of war, and an important example of early continental architecture. The Castillo de San Marcos is a National Monument in the United States, located in the city of St. Augustine, Florida, on the western side of Matanzas Bay. Visiting Castillo de San Marcos. The City of St. Augustine maintains a parking garage only a block away from the Castillo. Castillo de San Marcos Now. It was deactivated for military use in the 1930's. An Act of Congress restored the fort back to its original name in 1942 to preserve the history and significance of it.