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, the culmination of hundreds of years of military defense engineering.

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.

Given its light and porous nature, coquina would seem to be a poor choice of building material for a fort. However the Spanish had few other options; it was the only stone available on the northeast coast of La Florida. 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 balls fired at the walls of the Castillo burrowed their way into the rock and stuck there, much like a bb would if fired into Styrofoam. So the thick coquina walls absorbed or deflected projectiles rather than yielding to them, providing a surprisingly long-lived fortress.

^From nps.gov