Castillo de San Marcos The Castillo de San Marcos is the oldest masonry fort in the continental United States ( Castillo San Felipe del Morro in San Juan , Puerto Rico is older ) . Located on the shore of Matanzas Bay in the city of St . Augustine , Florida , construction began in 1672 , 107 years after the city ' s founding by Spanish Admiral and conquistador Pedro Menéndez de Avilés , when Florida was part of the Spanish Empire . After Britain gained control of Florida in 1763 pursuant to the Treaty of Paris , St . Augustine became the capital of British East Florida , and the fort was renamed Fort St . Mark until the Peace of Paris ( 1783 ) when Florida was transferred back to Spain . In 1819 Spain signed the Adams – Onís Treaty which ceded Florida to the United States in 1821 and the fort became a United States Army base which was renamed Fort Marion , in honor of American Revolutionary War hero Francis Marion . In 1942 the original name , Castillo de San Marcos , was restored by an Act of Congress . The fort was declared a National Monument in 1924 and after 251 years of continuous military possession , the fort was deactivated in 1933 and the site was turned over to the United States National Park Service . Castillo de San Marcos was twice besieged : first by English colonial forces led by Carolina Colony Governor James Moore in 1702 , and then by Georgia colonial Governor James Oglethorpe in 1740 . Possession of the fort has changed six times , all peaceful , amongst four different governments : the Spanish Empire , the Kingdom of Great Britain , the Confederate States of America and the United States of America ( Spain and the United States having possession two times each ) . Under United States control the fort was used as a military prison to incarcerate members of various Native American tribes starting with the Seminole - including the famous war chief Osceola - in the Second Seminole War - and members of various western tribes including Geronimo ' s band of Chiricahua Apache . The Native American art form known as Ledger Art had its origins at the fort during the imprisonment of members of the Plains tribes such as Howling Wolf of the Southern Cheyen . Construction . The European city of St . Augustine was founded by the admiral Pedro Menéndez de Avilés for the Spanish Crown in 1565 on the site of a former Native American village . Over the next 100 years , the Spanish built nine wooden forts for the defense of the town in various locations . The need for fortifications was recognized after it was attacked by Sir Francis Drake and his fleet of 20 ships in 1586 . Following the 1668 attack of the English pirate Robert Searle , Mariana , Queen Regent of Spain , approved the construction of a masonry fortification to protect the city . The Castillo is a masonry star fort made of a stone called " coquina " , Spanish for " small shells " , made of ancient shells that have bonded together to form a type of stone similar to limestone . Workers were brought in from Havana , Cuba , to construct the fort in addition to Native American laborers . The coquina was quarried from the ' King ' s Quarry ' on Anastasia Island in what is today Anastasia State Park across Matanzas Bay from the Castillo , and ferried across to the construction site . Construction began on October 2 , 1672 and lasted twenty - three years , with completion in 1695 . The fort has four bastions named San Pedro , San Agustín , San Carlos and San Pablo with a ravelin protecting the sally port . On the two landward sides a large glacis was constructed which would force any attackers to advance upward toward the fort ' s cannon and allow the cannon shot to proceed downslope for greater efficiency in hitting multiple targets . Immediately surrounding the fort was a moat which could be flooded to a depth of a foot during high - tide with seawater from Matanzas Bay prior to an attack via the use of floodgate s built into the seawall . Multiple embrasures were built into the curtain wall along the top of the fort as well as into the bastions for the deployment of cannon of various calibers . Infantry embrasures were also built into the walls below the level of the terreplein for the deployment of muskets by the fort ' s defenders . It was through one of these embrasures that twenty Seminoles held as prisoners would escape in 1837 . First English siege . In 1670 , Charles Town ( modern - day Charleston , South Carolina ) was founded by English colonists . As it was just two days ' sail from St . Augustine , the English settlement and encroachment of English traders into Spanish territory spurred the Spanish in their construction of a fort . In 1702 , English colonial forces under the command of Carolina Governor Governor James Moore embarked on an expedition to capture St . Augustine early in Queen Anne ' s War . The English laid siege to St . Augustine in November 1702 . About 1 , 500 town residents and soldiers were crammed into the fort during the two - month siege . The small English cannon had little effect on the walls of the fort , because the " coquina " was very effective at absorbing the impact of the shells . The siege was broken when the Spanish fleet from Havana arrived , trapping some English vessels in the bay . The English burned their ships to prevent them from falling under Spanish control , and then marched overland back to Carolina . The town of St . Augustine was destroyed , in part by the Spanish and in part by the English , as a result of the siege . Second period of construction . Beginning in 1738 , under the supervision of Spanish engineer Pedro Ruiz de Olano , the interior of the fort was redesigned and rebuilt . Interior rooms were made deeper , and vaulted ceilings replaced the original wooden ones . The vaulted ceilings allowed for better protection from bombardments and allowed for cannon to be placed along the gun deck , not just at the corner bastions . The new ceilings required the height of the exterior wall to be increased from 26 to . Second British Siege . Spain and Britain were rivals in Europe . Because the two countries had initiated empires in the New World , their heated rivalry continued . In 1733 a British vessel , the " Rebecca " , commanded by Captain Robert Jenkins , was seized in the Caribbean by the Spanish coast guard . Suspecting that the British had been trading illegally with Spanish colonies ( which was forbidden by both Spain and Britain ) , the Spanish searched the ship . A fight broke out between the Spanish and British sailors . In the skirmish , Jenkins had his ear cut off by a Spanish officer , who picked it up and said " Take this to your king and tell him that if he were here I would serve him in the same manner ! " When Jenkins reported the incident to British authorities , they used it as a catalyst to declare war on Spain in 1739 . The war was called the War of Jenkins Ear . After British Admiral Edward Vernon scored a huge victory at Portobelo , General James Oglethorpe , the founder of Georgia , was quick to imitate him in North America . In June 1740 , Oglethorpe and an English fleet of seven ships appeared off St . Augustine . As in the 1702 siege , three hundred soldiers and 1 , 300 residents found refuge within the Castillo ' s walls . For 27 days the British bombarded the Castillo and St . Augustine . Realizing his cannon were not affecting the Castillo ' s walls , Oglethorpe decided to starve the people of St . Augustine by blockading the inlet at the Matanzas River and all roads into St . Augustine . No supplies could reach the city . With morale and supplies low for his own forces , Oglethorpe had to retreat . To protect the fort from future blockades and siege Fort Matanzas was built . British occupation . In 1763 , the British managed to take control of the Castillo but not by force . As a provision of the Treaty of Paris ( 1763 ) after the Seven Years War , Britain gained all of Spanish Florida in exchange for returning Havana and Manila to Spain . On July 21 , 1763 , the Spanish governor turned the Castillo over to the British , who established St . Augustine as the capital of the province of East Florida , established by the Royal Proclamation of 1763 . The British made some changes to the fort , and renamed it Fort St . Mark . As Great Britain was the dominant power in North America , they were not worried about keeping the fort in top condition . This attitude prevailed until the outbreak of the American Revolutionary War . The fort was used as a military prison during the war . Among those imprisoned was Christopher Gadsden , the Lieutenant governor of South Carolina . He was also a delegate to the Continental Congress and a brigadier general in the Continental Army during the war . He was released after 11 months . Improvements were begun on the fort , in keeping with its new role as a base of operations for the British in the South . The gates and walls were repaired , and second floors were added to several rooms to increase the housing capacity of the fort . The Castillo saw action during the American Revolution mainly as a prison , although St . Augustine was targeted by several aborted expeditions from Georgia . Several revolutionary fighters who had been captured in Charleston were held there when it was taken by the British . The Spanish declared war on Britain in 1779 , drawing off forces from Fort St . Mark and keeping the British occupied . Bernardo de Gálvez , governor of Spanish Louisiana , attacked several British - held cities in West Florida , capturing all of them . The only major British operation that used troops from St . Augustine was the poorly - coordinated but successful capture of Savannah , Georgia ; the city was taken by troops from New York before those from St . Augustine arrived . At the end of the war , the 1783 Treaty of Paris called for the return of Florida to Spain . On July 12 , 1784 , Spanish troops returned to St . Augustine . Second Spanish period . When Spain regained control over Florida they found a much changed territory . Many Spaniards had left Florida after the hand over to Britain , and many British citizens stayed after the hand over back to Spain . Many border problems arose between Spanish Florida and the new United States . Spain changed the name of the fort back to the Castillo de San Marcos , and continued to build upon the improvements that Britain had made to the fort in an effort to strengthen Spain ’ s hold on the territory . However , due to increased pressure from the United States and several other factors , in 1819 , Spain signed the Adams – Onís Treaty , ceding Florida to the United States , which was transferred in 1821 . First United States period . Upon receiving the fort from Spain , the Americans changed its name to Fort Marion . It was given that name to honor the legendary American Revolutionary War hero General Francis Marion " The Swamp Fox " . Structurally , they made few changes to the fort during this time . Many storerooms were converted to prison cells on account of their heavy doors and barred windows . Also , part of the moat was filled in and transformed into an artillery battery as part of the American coastal defense system . The original Spanish seawall was dismantled to ground level and a new seawall constructed immediately adjacent to the seaward side of the original . At this time a hotshot furnace was also built in the filled in section of the moat behind the newly built water battery . Cannonballs were heated in the furnace to fire at wooden enemy ships . In October 1837 , during the Second Seminole War , Seminole chief Osceola was taken prisoner by the Americans while attending a peace conference under a flag of truce . He was imprisoned in the fort along with his followers , including Uchee Billy , King Philip and his son Coacoochee ( Wild Cat ) . Uchee Billy was captured on September 10 , 1837 , and he died at the fort on November 29 . His skull was kept as a curio by Dr . Frederick Weedon , who also decapitated Osceola after his death in Fort Moultrie and kept the head in preservative . On the night of November 19 , 1837 Coacoochee along with Talmus Hadjo , 16 other Seminole braves and two Seminole women escaped from Fort Marion by squeezing through the eight inch ( 203 mm ) opening of the embrasure located high in their cell and sliding down a makeshift rope into the moat . They made their way to their band ' s encampment where the Tomoka River meet the Atlantic . Due to their treatment they vowed to continue fighting and prolonged the war for four more years . In the past " Coacoochee ' s Cell " from which he escaped was part of the official lore of the fort . Confederate States period . In January 1861 , Florida seceded from the United States in the opening months of the American Civil War . Union troops had withdrawn from the fort , leaving only one man behind as caretaker . In January 1861 , Confederate troops marched on the fort . The Union soldier manning the fort refused to surrender it unless he was given a receipt for it from the Confederacy . He was given the receipt and the fort was taken by the Confederacy without a shot . Most of the artillery in the fort was sent to other forts , leaving only five cannons in the water battery to defend the fort . The Saint Augustine Blues , a militia unit formed in St Augustine were enrolled into the Confederate Army at Ft . Marion on August 5 , 1861 . They were assigned to the recently organized Third Florida Infantry as its Company B . More than a dozen former members of the St . Augustine Blues are buried in a row at the city ' s Tolomato Cemetery . The fort along with the rest of the city of St Augustine was reoccupied by Union troops after acting mayor Cristobal Bravo officially surrendered the city to Union Navy fleet commander C . R . P Rodgers on March 11 , 1862 . The Confederate forces left the city the previous evening in anticipation of the arrival of the Union fleet under the command of Commodore Dupont . Second United States period . The fort was taken back by Union forces on March 11 , 1862 , when the USS " Wabash " entered the bay , finding the city evacuated by Confederate troops . The city leaders were willing to surrender in order to preserve the town , and the city and the fort were retaken without firing a shot . Throughout the rest of the fort ' s operational history , it was used as a military prison . Beginning in 1875 , numerous Native American prisoners were held at the fort in the aftermath of the Indian Wars in the west . Many would die at the fort . Among the captives were Chief White Horse of the Kiowa , and Chief Grey Beard of the Southern Cheyenne . During this period , Richard Henry Pratt , a Civil War veteran , supervised the prisoners and upgraded the conditions for them . He developed ways to give the men more autonomy and attempted to organize educational and cultural programs for them . They became a center of interest to northerners vacationing in St . Augustine , who included teachers and missionaries . Pratt recruited volunteers to teach the Indian prisoners English , Christian religion , and elements of American culture . He and most US officials believed that such assimilation was needed for the Indians ' survival in the changing society . The men were also encouraged to make art ; they created hundreds of drawings . Some of the collection of Ledger Art by Fort Marion artists is held by the Smithsonian Institution . It may be viewed online . Encouraged by the men ' s progress in education , citizens raised funds to send nearly 20 of the prisoners to college after they were released from Ft . Marion . Seventeen men went to the Hampton Institute , a historically black college . Others were sponsored and educated in New York state at private colleges . Among the latter were David Pendleton Oakerhater , as he became known , who was sponsored by US Senator Pendleton and his wife . He studied and later was ordained as an Episcopal priest . He returned to the West to work as a missionary with Indian tribes . He was later recognized by the Episcopal Church as a saint . Based on his experience at Fort Marion , Pratt recommended wider education of Indian children , a cause which Senator Pendleton embraced . He sponsored a bill supporting this goal , and the US Army offered the Carlisle Barracks in central Pennsylvania as the site of the first Indian boarding school . Its programs were developed along the industrial school model of Hampton and Tuskegee Institute s , which officials thought appropriate to prepare Native Americans for what was generally rural reservation life . Through the early 20th century , the government founded 26 other Indian boarding schools , and permitted more than 450 boarding schools run by religious organizations . From 1886 - 1887 , approximately 491 Apaches were held prisoner at Fort Marion ; many were of the Chiricahua and Warm Springs Apache bands from Arizona . There were 82 men and the rest were women and children . Among the men , 14 , including Chatto , had previously been paid scouts for the US Army . Among the Chiricahua were members of the notable chief Geronimo ' s band , including his wife . Geronimo was sent to Fort Pickens , in violation of his agreed terms of surrender . While at the fort , many of the prisoners had to camp in tents , as there was not sufficient space for them . At least 24 Apaches died as prisoners and were buried in North Beach . In 1898 , over 200 deserters from the Spanish - American War were imprisoned at the fort . This marked one of the last uses of the fort as an operational base . In 1900 , the fort was taken off the active duty rolls after 205 years of service under five different flags . In 1924 , the fort was designated a National Monument . In 1933 it was transferred to the National Park Service from the War Department . In 1942 , in honor of its Spanish heritage , Congress authorized renaming the fort as " Castillo de San Marcos " . As an historic property of the National Park Service , the National Monument was listed on the National Register of Historic Places ( NRHP ) on October 15 , 1966 . The National Park Service manages the Castillo together with Fort Matanzas National Monument . In 1975 , the Castillo was designated an Historic Civil Engineering Landmark by the American Society of Civil Engineers . Since being transferred to the Park Service , the Castillo has become a popular tourist attraction . It occupies 2 . 5 acres ( 10 , 100 m² ) in downtown St . Augustine , Florida .