Castillo de San Marcos

St. Augustine's most historically significant structure is the Castillo de San Marcos, a fort constructed by the Spanish between 1672 and 1695.

The need to fortify the tiny garrison town was understood as early as 1586, when it was attacked by Sir Francis Drake, an English corsair whose fleet of twenty ships and two thousand men sacked and burned the town.

Later, in 1668, the English pirate Robert Searles assaulted and plundered the settlement. At the same time the threat of English colonization in the north had significantly increased, and Queen Mariana of Spain authorized the construction of a stone fortification.

The fort was built of coquina, a type of shell stone indigenous to the area and quarried from Anastasia Island.

The Castillo has never been conquered despite attacks by English General James Oglethorpe who attempted to subdue the town in 1740. Firing from the tip of Anastasia Island, he found his cannonballs were no match for the unusual consistency of coquina which absorbed the blast rather than crumbling.

Shortly after Florida became a territory of the United States, the fort was renamed Fort Marion in honor of Revolutionary War General Francis Marion. It retained that name from 1825 to 1942, when the Castillo appellation was restored.

In the 1870s and 1880s, the fort housed Indians detained by the U.S. Government. The Castillo and Fort Matanzas have been under the auspices of the National Park Service since 1933. The fort is open to the public daily, except for Christmas.

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

The Castillo de San Marcos, built 1672-1695, served primarily as an outpost of the Spanish Empire, guarding St. Augustine, the first permanent European settlement in the continental United States, and also protecting the sea route for treasure ships returning to Spain. Although the Castillo has served a number of nations throughout its history, it has never been taken by military force. During the 18th century, the Castillo went from Spanish control to British and back to the Spanish, all by treaty. The Spanish remained in power in Florida until the area was purchased by the United States in 1821. Called Fort Marion at this time, the Castillo was used by the US army until 1899. The Castillo was made a national monument in 1924..