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)

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 Castle?

A very interesting question, and there really isn't a 100% clear answer. According to Wikipedia, "the definition of a castle accepted amongst academics is 'a private fortified residence'." Merriam-Webster dictionary says a castle is "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 Castillo de San Marcos fits all of those definitions in one way, or at one time, or another. When it was first built, the governor of St. Augustine resided inside the building, which would make it a "private fortified residence." 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!). And it also has thick walls, battlements, towers, and a moat.

Most of the fortifications the Spanish built in the New World were named Castillos. Perhaps it is a hold-over from medieval times, meant to inspire their people and instill fear in their enemies? The Spanish were fond of decoration and embellishment in the physical designs of their fortresses; they may have felt the same about their names. As of yet, there has been no documentation found explaining exactly why they chose to use Castillo rather than Fuerte or Fortaleza. However, it is interesting to note that the wooden fort that preceded the current stone one was also called Castillo de San Marcos.

Also interesting is the fact that it does later become referred to as a fort. 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 for good.


Last updated: June 6, 2017

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