The interior of Fort Warren's demilune is a cavalcade of charmingly dank spaces.

During the Civil War (1861-1865) prisoners were held at Fort Warren, some within the demilune. Outside the front of this structure is a sign describing the adventures of a Union deserter named Private Charles Sawyer, who escaped from incarceration within this very demilune by piling hot coals along the edges of the firing loophole in his cell, which over time caused the granite to crack, allowing him enough room to wriggle out. Sawyer leapt into the channel and headed for a nearby island, but the unpredictable waters of Boston Harbor were treacherous, and he was captured by a passing schooner. Which is quite a relief, because I would feel monumentally threatened if THAT guy had made it to one of the other barren islands in Boston Harbor.