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From the Civil War through World War II, Fort Warren provided the principal defense of Boston Harbor. Military engineer Sylvanus Thayer oversaw the fortification of the New England coast from 1833 through 1863, creating here his most important commission. Fort Warren served as a prison for key Confederate leaders, including Alexander Stephens, vice president of the Confederacy, immediately following the Civil War. A massive single-story structure of Quincy granite, the fort is pentagonal in plan, surrounded by a deep redoubt and enclosing a vast parade ground. In 1958, the federal government conveyed the fort to the Metropolitan District Commission (now the State Department of Conservation and Recreation), who provides boat service to Georges Island in the summers.