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. Originating in Italy in the 15th century the particular star shaped design of the Castillo is a result of architecture adapting to technology.
A monument not only of stone and mortar but of human determination and
endurance, the Castillo de San Marcos symbolizes the clash between
cultures which ultimately resulted in our uniquely unified nation.
Still resonant with the struggles of an earlier time, these original
walls provide tangible evidence of America’s grim but remarkable
history (all excerpts taken here).
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 made of crushed seashells.
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.
Spanish St. Augustine was the first of only three walled cities ever
constructed in North America; the other two are (English) Charleston,
South Carolina and (French) Quebec City, Canada. Castillo de San Marcos National Monument, Florida.