Two forts protected Philadelphia from potential Confederate attack. Located along the Delaware River just south of downtown, Fort Mifflin was primarily a prison, housing both Confederate prisoners of war and Union soldiers and civilians accused of war crimes. Further south along the river, Fort Delaware served as the primary defense of the ports of Philadelphia and Wilmington, Delaware. The Union also maintained a large prisoner of war camp at this fort.
Image shown: Fort Mifflin, A Prison for Deserters. In: Philadelphia in the Civil War, 1861-1865, by Frank H. Taylor.