Were this Fort Jefferson, this would be the casemate where Dr. Samuel Mudd cooled his heels after (maybe) participating in Abraham Lincoln's assassination in 1865 (see that cheery place here). But we're in Fort Barrancas, which was not, as far as I've gathered, ever used as a prison. Some of the guys stationed there may have felt like they were in prison, but that was their own fault for joining the military or allowing themselves to be drafted.