Castillo de San Marcos National Monument preserves the oldest masonry fortification in the continental United States, completed by Spanish settlers in 1705 after 23 years of construction. Built from coquina, a local shell-based stone designed to absorb cannon fire, the fortress successfully repelled attacks in 1702, 1728, and 1740. Located on 20.5 acres in Saint Augustine, Florida, the fort has protected Spain's colonial settlement for centuries and remained strategically significant until 1900. Visitors can explore the historic structure, watch cannon demonstrations, and participate in living history programs and reenactments throughout the year. Admission is $15 for adults ages 16 and older.
