Architecture & Construction

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 of St. Augustine.

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.

1770's view of the Castillo de San Marcos' star shaped bastions.
Map detail showing the design of the Castillo and its outerworks.

Bastion System

Originating in Italy in the 15th century the particular star shaped design of the Castillo is a result of architecture adapting to technology. The change in warfare brought about by black powder weapons created new types of defensive structures adapted both to withstand or avoid the impact of cannon projectiles and to effectively mount cannons to repel any attackers. Of the major architectural variations the "bastion system," named for the projecting diamond or angle shaped formations added onto the fort walls, was the most commonly and effectively used.

The coquina quarries on Anastasia island supplied the Spanish with building materials for over 200 years.
The Spanish Canterra on Anastasia Island

The Stone That Saved Spanish Florida

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.

Is it a Fort or a Castle?

A very interesting question. The short answer is: Both. Both is good. It really depends on who used it and what purpose the structure served. Merriam-Webster Dictionary defines a castle as "a large fortified building or set of buildings; a retreat safe against intrusion or invasion." Oxford English Dictionary says it is "a large building, typically of the medieval period, fortified against attack with thick walls, battlements, towers, and in many cases a moat." The Real Academia Española states it is "a strong place, surrounded by walls, bastions, moats, and other fortifications."

The Castillo de San Marcos fits all of those definitions in one way, or at one time, or another. It is most definitely a large, fortified building that provided a retreat safe against invasion for the people of St. Augustine; at one time, it housed over 1,500 people for 51 days while the English laid siege! It also has thick walls, battlements, towers, and a moat.

Many of the fortifications the Spanish built in the New World were called castillos. It is interesting to note that at least two of the wooden forts that preceded the current stone one in St. Augustine were also called "Castillo de San Marcos." The building does not need to be made of stone to be classified as a castillo.

Later in the park's story, our Castillo does become referred to as a fort. While the Real Academia Española states a fortress is a "fortified enclosure, such as a castle, citadel," Merriam-Webster defines a fort as "a fortified place occupied only by troops and surrounded with such works as a ditch, rampart, and parapet." When the British gained Florida through the 1763 Treaty of Paris, they renamed the building Fort Saint Mark, and the United States Army decided in 1825 to call it Fort Marion. Under those occupations, it was indeed used for purely military function. The British and the Americans did not plan to use it as a place of refuge for the citizens of St. Augustine; they both used as barracks, for military storage, and a few times as a military prison.

The National Park Service and United States Congress decided to restore its original name in 1942, in honor of its unique Spanish history, so it went back to Castillo de San Marcos.

Why did each group choose to change what this imposing structure should be called? Was it due to form and function? Or did it just roll off the tongue more easily for the new owners? What do you think?


Last updated: October 22, 2023

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