4. Castillo de San Marcos (Florida)
Built in 1672, Castillo de San Marcos stands as the oldest masonry fort in the continental United States. Located in St. Augustine, its distinctive coquina stone walls and star-shaped layout reveal centuries of Spanish colonial and military history. Much like the renowned fortifications of San Juan, Puerto Rico, this enduring fortress has withstood sieges and storms, offering visitors a vivid sense of the region’s tumultuous past and enduring strategic importance.
