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 (The other is Fort Matanzas National Monument 14 miles south)
The fortress itself is both a product of and evidence to the multitude of forces both political and technological that created the competition for empire during the colonial era. But above all the Castillo is an enduring legacy of the craftsmanship and skill of the engineers, artisans and labourers who built it.