Fort Warren ( Massachusetts ) Fort Warren is a historic fort on the Georges Island at the entrance to Boston Harbor . The fort is pentagon al star fort , made with stone and granite , and was constructed from 1833 – 1861 , completed shortly after the beginning of the American Civil War . Fort Warren defended the harbor in Boston , Massachusetts , from 1861 through the end of WWII , and during the Civil War served as a prison for Confederate officers and government officials . The fort remained active through the Spanish - American War and World War I , and was re - activated during World War II . It was permanently decommissioned in 1947 and is now a National Historic Landmark and a tourism site . It was named for Revolutionary war hero Dr . Joseph Warren who sent Paul Revere on his famous ride . Dr . Warren was killed at the Battle of Bunker Hill . History . Fort Warren was built from 1833 to 1861 and was completed shortly after the beginning of the American Civil War . The Army engineer in charge during the bulk of the fort ' s construction was Colonel Sylvanus Thayer best known for his tenure as Superintendent of the United States Military Academy at West Point , New York . During the Civil War , the island fort served as a prison for captured Confederate army and navy personnel , elected civil officials from the state of Maryland , as well as Northern political prisoners . James M . Mason and John Slidell , the Confederate diplomats seized in the " Trent " affair , were among those held at the fort . Military officers held at Fort Warren include Richard S . Ewell , Isaac R . Trimble , John Gregg , Adam " Stovepipe " Johnson , Simon Bolivar Buckner , Sr . , and Lloyd Tilghman . High ranking civilians held at Fort Warren include Confederate Vice President Alexander H . Stephens , and Confederate Postmaster General John Henninger Reagan . The prison camp had a reputation for humane treatment of its detainees . When the camp commander ' s son , Lieutenant Justin E . Dimick , left Fort Warren for active duty in the field with the Second U . S . Artillery , he was given a letter from Confederate officers in the camp urging good care should he be captured . ( He was later mortally wounded at Chancellorsville in May , 1863 . ) The famous Union marching song " John Brown " was written at the fort using a tune from an old Methodist camp song . The song was carried to the Army of the Potomac by the men of the " Webster Regiment " ( 12th Massachusetts Infantry ) who had mustered in at Fort Warren . Julia Ward Howe heard this song while visiting Washington DC . At the suggestion of her minister , Howe was encouraged to write new words . " The Battle Hymn of the Republic " , which was initially published as a poem , was later matched with the melody of the " John Brown " song and became one of the best remembered songs of the Civil War era . ( See also : List of Civil War POW Prisons and Camps ) After the Civil War . Fort Warren remained active through the Spanish - American War and World War I . The fort was modified in the late 1890s through the beginning of the twentieth century to accommodate the newer rifled ordnance then being developed for coastal defense . During World War II , the fort served as a control center for Boston Harbor ' s south mine field , a precaution taken in anticipation of potential attacks by Kriegsmarine U - boat s . At that time , Fort Warren was staffed by personnel of the 241st Coast Artillery ( Harbor Defense ) , a Massachusetts National Guard unit that was federalized in September , 1940 . Fort Warren was permanently decommissioned after 1950 . Decommissioning and opening to the public . Fort Warren was owned by the U . S . federal government until 1958 , when the state obtained possession from the General Services Administration . In 1961 , the fort was reopened to the public after initial restoration efforts . Today , the Massachusetts Department of Conservation and Recreation maintains and administers the fort , which is the centerpiece of the Boston Harbor Islands National Recreation Area . The fort is reachable by ferry from downtown Boston , Hingham , or Hull to Georges Island . Transfers are then available for those who wish to visit some of the other Harbor Islands . The fort is typically open from early or mid May through Columbus Day weekend . DCR Rangers offer guided tours or you may explore on your own . An information booth just outside the sally port ( the main entrance to the fort ) posts information about available activities . The island offers a well - stocked snack bar , water fountains , and a large number of composting toilets . There is also a museum located in the old mine storehouse ( the red brick building opposite the ferry dock ) , a number of picnic tables , and a children ' s play structure . The tops of several of the bastions ( the walls ) and several of the casemates and magazines beneath them are open to visitors .