The Castillo de San Marcos is the oldest masonry fort in the continental United States (Castillo San Felipe del Morro in San Juan, Puerto Rico is older). Located on the shore of Matanzas Bay in the city of St. Augustine, Florida, construction began in 1672, 107 years after the city's founding by Spanish Admiral and conquistador Pedro Menéndez de Avilés, when Florida was part of the Spanish Empire.