Built by the Spanish in St. Augustine to defend Florida and the Atlantic trade route, Castillo de San Marcos National Monument preserves the oldest masonry fortification in the continental United States and interprets more than 450 years of cultural intersections.

The Castillo de San Marcos in St. Augustine, Florida, is the oldest masonry fort in the continental United States. Construction by the Spanish began in 1672, making it over 350 years old. It is unique because it is built of coquina, a rare limestone made of broken sea shells, which made the walls "elastic" enough to absorb cannonballs rather than shattering under impact. While there are older "sites" where wooden forts once stood (such as Fort Caroline), the Castillo is the oldest standing structure of its kind. In 2026, it is a National Monument managed by the Park Service. It is important to note that if you count US territories, La Fortaleza in Puerto Rico (built 1533) is technically older, but for the mainland US, the Castillo de San Marcos remains the undisputed "grandfather" of American military fortifications, having never been taken by force in battle.