The Castillo de San Marcos is unique in North American architecture. As the only extant 17th century military construction in the country and the oldest masonry fortress in the United States it is a prime example of the "bastion system" of fortification, named for the projecting diamond or angle shaped formations added onto the fort walls.⁣ ⁣ It is also unique for the material used in its construction. The Castillo is one of only two fortifications in the world built out of a semi-rare form of limestone called coquina (The other is Fort Matanzas National Monument 14 miles south) Given its light and porous nature, coquina would seem to be a poor choice of building material for a fort. However, coquina's porosity turned out to have an unexpected benefit. Because of its conglomerate mixture coquina contains millions of microscopic air pockets making it compressible. A cannon ball fired at more solid material, such as granite or brick would shatter the wall into flying shards, but cannon ...