The Castillo de San Marcos (Spanish for "St. Mark's Castle") is the oldest masonry fort in the continental United States; it is located on the western shore of Matanzas Bay in the city of St. Augustine, Florida. Though built in part by black slaves owned by the Spanish, the fort later served as one of the first entry points of fugitive slaves from British North America into Spanish Florida, where they were freed by the colonial authorities. This quickly led to the first free Black settlement in the future United States (Fort Mose, formed just north of St Augustine).