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Castillo de San Marcos

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1671–1695, Ignazio Daza, engineer. 1 S. Castillo Dr.
  • View of San Augustin Bastion, 1955

The Castillo de San Marcos in St. Augustine is often cited as the oldest military structure in the United States and the oldest masonry fortress, as well as being one of just two buildings in the world to be constructed out of coquina limestone, a rare formation of limestone composed of very small shells. With these distinctions, Castillo de San Marcos touches the rich geopolitical history of Florida and the Americas as it connects to a local building material, which literally forms part of the bedrock of the state. St. Augustine was located on the site of the Timucua settlement of Seloy, and the current fortress follows nine prior wooden fortifications built on the same site since the founding of the city in 1565.

Nearly a century after the “discovery” by Ponce de Leon and the conquest of Florida for Spain, the British founded their first permanent settlement in Jamestown, Virginia, in 1607 and further north in the Plymouth Colony in 1620. With the establishment of the Carolina Charters of 1663 and 1665 and the founding of Charleston in 1670, it became abundantly clear that conflict between Spain and Britain was inevitable. A century and a half after Ponce de Leon’s arrival in the area, Castillo de San Marcos was constructed to provide Spain more robust and indestructible protection for Florida and, more importantly, shipping routes along the Gulf Stream that passed through the Gulf of Mexico, the Straits of Florida, and along the coast of Florida before crossing the Atlantic Ocean.

Developments in warfare, particularly in the areas of more powerful gunpowder and larger cannons required responses in fortification architecture. In the late 1400s and early 1500s, a number of European engineers, most famously including the Italian Leonardo da Vinci, invented a new bastion system of fortification that featured thick-walled, star-shaped prominences designed to allow those inside the fortress to get closer to and better angles on the enemy while affording only glancing targets for offensive strategies. These new fortresses also featured ravelins, which were detached fortifications connected to the fortress by bridges. As designed by Havana-based military engineer Ignazio Daza, the Castillo de San Marcos had all the markings of a modern fortification. The use of the soft, local coquina limestone proved to have additional advantages. While harder stone used in traditional fortresses tended to splinter and fracture into deadly projectiles when struck by a cannon ball, the soft coquina limestone simply absorbed the cannonballs much like a bale of hay absorbs arrowheads. This resulted in considerably less casualties of those defending the fort.

If earlier designs followed the contours of rocky landscapes—like those of Fortaleza Ozama (started in 1502) in Santo Domingo, Dominican Republic, or Castillo San Felipe del Morro in San Juan, Puerto Rico (1539), or in Havana, Cuba (1589)—the Castillo de San Marcos was designed for a relatively flat site and uses an almost-mathematical rectangular regularity in the dimensions of each side. Surrounded by a moat, the fort has a central courtyard with bastions at each of the four corners and the ravelin guarding the southern flank. Individual rooms open directly onto the courtyard and all vertical circulation takes place along the southern and eastern sides.

The first test of the fortress came in 1702 when South Carolina’s Governor, James Moore laid siege to St. Augustine during Queen Anne’s War. The siege lasted fifty-eight days before the British retreated, having burned St. Augustine. In preparation for a second failed siege, this one by Georgia Governor James Ogelthorpe in 1740, the Governor of Florida, Manuel de Montiano raised the height of the walls and added arched casements to strengthen the fortress. This brought the architecture of the Castillo to the form in which it may still be seen today.

The fort was declared a National Monument in 1924 and was decommissioned in 1933. The National Park Service has operated the site since that time.


Mauncy, Albert C. The Building of Castello de San Marcos: National Park Service Interpretive Series, History No. 1. Washington, D.C.: Government Printing Office, n.d.

National Park Service. Castillo de San Marcos : a guide to the Castillo de San Marcos National Monument, Florida. Washington, D.C.: U.S. Department of the Interior, 1993.

Writing Credits

John Stuart
David Rifkind
John Stuart



  • 1912

    Design and construction

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John Stuart, "Castillo de San Marcos", [St. Augustine, Florida], SAH Archipedia, eds. Gabrielle Esperdy and Karen Kingsley, Charlottesville: UVaP, 2012—,

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