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This is one of Fort Warren's unique features: a demilune. What is a demilune you ask? It is a semi-circular or semi-lunar shaped masonry defense which was attached to the main fort to defend against an amphibious assault on George's Island in Boston Harbor. The well-built fort served the military for 100 years as a patrol point and training ground. It gained a favorable reputation for the humane treatment of Confederate prisoners during the Civil War and was never fired upon.
The dark grey stone with the chisel-edge border, known as quarry-face finish, came from Quincy. Stone for the fort's construction also came from Cape Anne and quarries in Vermont.