A few years after Menendez founded St. Augustine in 1565, a wooden fort was built near the site of the present Castillo. Since Spain controlled Florida, officials initially saw no reason to build strong forts there. In the years that followed, however, more British settlements were established along the east coast of North America.

Soon British pirates were attacking Spanish towns and ships. In 1586, Sir Francis Drake, an English explorer, captured the fort and burned the small town to the ground. In 1668 pirates attacked the town of St. Augustine and the Spanish colonists were barely able to fight them off.

Queen Mariana of Spain, worried about these attacks, ordered a masonry fort built at St. Augustine to defend the small colony and protect the Spanish ships. Construction of the Castillo began on October 2, 1672 and was essentially complete by 1695. Ignacio Daza, a Spanish engineer living in Cuba, designed the fortification. Local Native Americans, black slaves and Spanish soldiers worked alongside skilled stone masons from Cuba and Spain. Hand-cut coquina blocks were quarried from nearby Anastasia Island. This soft limestone was formed from naturally cemented seashells. Laborers then applied tabby, composed primarily of lime and oyster shells, as mortar to hold the blocks together.

From above, San Marcos resembles a star with four large points(bastions). It contains more than 20 rooms which once housed weapons, medical supplies, rations and soldiers. The fort's perimeter measures 320 feet. It boasts 30-foot high walls, which range in thickness from 12 to 17 feet on the bottom to five feet at the top. A 40-foot-wide moat, lined with coquina, surrounds the fortress. The Spanish term "castillo"; fits it well since the fort's moat and drawbridges mimic those of Europe's medieval castles, and the coquina stone absorbed cannon balls.

After Spain ceded Florida to the United States in 1821, the Americans changed the name of the Castillo to Fort Marion in 1825. On the eve of the Civil War, the U.S. military presence had largely been withdrawn from St. Augustine and Fort Marion's caretaker garrison consisted of one ordnance sergeant.

As the Secession Convention was meeting in Tallahassee in early January 1861, Governor Madison S. Perry ordered the seizure of Fort Marion. On January 7, 1861, state militia from St. Augustine and Fernandina seized the fort without violence.

During the remainder of 1861, a number of Artillery pieces were removed from Fort Marion to reinforce Confederate troops at Fort Clinch near Fernandina, the St. Johns Bluff, and outside the state. Confederate forces occupied Fort Marion until March 1862, when they evacuated St. Augustine.

Union forces occupied the abandoned Fort Marion, strengthened its defenses, and remained at the fort for the duration of the war. No Confederate attempt was made to retake the fort.

In 1933, Fort Marion was transferred from the War Department to the National Park Service, and, in 1942, the fort's name was restored to Castillo de San Marcos. The Castillo de San Marcos National Historic Landmark represents the oldest remaining European fortification in the continental United States. The masonry fort and its surrounding land comprise 25 acres in historic downtown St. Augustine, the oldest continuously occupied European settlement in North America.

Unique among national parks, the Castillo de San Marcos, a national monument, encompasses nearly 450 years of history and culture. Historical and cultural influences of various groups associated with the site are preserved and interpreted. Significant to Native American heritage is the use of the fort as a prison cell for Osceola before he was moved to Fort Sumter, and as a prison cell for Apache warriors in the late 1800s.

Information provided by Florida Department of State.