National monument, the Castillo de San Marcos, is the oldest masonry Fort in the United States and the only standing 17th century military construction. It is also one of two forts built of coquina, a soft limestone composed of broken shells. This native material gave the Spanish a special advantage. Instead of cracking or collapsing, coquina walls absorbed or deflected enemy projectiles allowing the Spanish to guard their possessions and keep their people safe. This unique craftsmanship has been standing since 1695 and is continually admired by tourists today. The fort sits on a 20.5 acre national park where tourists can step inside the walls of the fort, hear cannons fly and attend live reenactments.