Fort Matanzas National Monument Fort Matanzas National Monument was designated a United States National Monument on October 15 , 1924 . The monument consists of a 1740 Spanish fort called Fort Matanzas , and about 100 acres ( 0 . 4 km² ) of salt marsh and barrier island s along the Matanzas River on the northern Atlantic coast of Florida . It is operated by the National Park Service in conjunction with the Castillo de San Marcos National Monument in the city of St . Augustine . History . Fort Matanzas was built by the Spanish in 1742 to guard Matanzas Inlet , the southern mouth of the Matanzas River , which could be used as a rear entrance to the city of St . Augustine . Such an approach avoided St . Augustine ' s primary defense system centered at Castillo de San Marcos . In 1740 , Gov . James Oglethorpe of Georgia used the inlet to blockade St . Augustine and launch a thirty - nine day siege . St . Augustine endured the siege but the Spanish realized the need to protect the inlet . Under Gov . Manuel de Montiano ' s orders , construction began in 1740 and was completed in 1742 . Engineer Pedro Ruiz de Olano , who had worked on additions to the Castillo de San Marcos , designed the fortified observation tower . Convicts , slaves , and troops from Cuba were used as labor to erect the structure , which was sited on present - day Rattlesnake Island and had a commanding position over Matanzas Inlet . The fort , known to the Spanish as " Torre de Matanzas " ( Matanzas Tower ) , is constructed of coquina , a common shellstone building material in the area . It is long on each side with a high tower . The marshy terrain was stabilized by a foundation of pine pilings . The standard garrison of the fort was one officer in charge , four infantrymen , and two gunners , though more troops could be stationed if necessary . All soldiers at Fort Matanzas served on rotation from their regular duty in St . Augustine . Five cannon were placed at the fort — four six - pounders and one eighteen - pounder . All guns could reach the inlet , which at the time was less than half a mile away . In 1742 , as the fort was nearing completion , the British under Oglethorpe approached the inlet with twelve ships . Cannon fire drove off the scouting boats and the warships left without engaging the fort . This brief encounter was the only time Fort Matanzas fired on an enemy . Spain lost control of Florida with the Treaty of Paris , 1763 , and regained control with the Treaty of Paris , 1783 . With the Spanish Empire falling apart , Spain spent little effort maintaining the fort after this time . When the United States took control of Florida in 1821 , the fort had deteriorated to the point where soldiers could not live inside . The United States never used the fort and it became a ruin . Fort Matanzas was named for the inlet , which acquired its name after the executions , or " matanzas " ( Spanish : slaughters ) , on its north shore , of Jean Ribault and his band of Huguenot Frenchmen , the last of the Fort Caroline colonists , by the Spanish in 1565 . Restoration and modern use . In 1916 , the U . S Department of War began a major restoration of the badly deteriorated fort . By 1924 , three vertical fissures in the wall were repaired and the structure was stabilized ; in the same year , National Monument status was proclaimed . Fort Matanzas was transferred from the War Department to the National Park Service on August 10 , 1933 . As a historic area under the Park Service , the National Monument was listed on the National Register of Historic Places on October 15 , 1966 . The Fort Matanzas National Monument Headquarters and Visitor Center were added separately to the National Register on December 31 , 2008 , as significant works of National Park Service architectural design .