The ingenious iron shutter system devised by none other than Joseph Totten. These shutters were designed to prevent American gun crews from being sniped from without while serving their piece: Before firing the gun, the spring-loaded bolts at the bottom of each door would be manually disengaged. The shutters would be left closed, but were so cleverly balanced that the gases emanating from the gun barrel ahead of the projectile would swing them open, and then back shut again, automatically engaging the locking bolts, after the projectile was on its way to smite the enemy of the Republic. Brilliant, and just what one would expect from General Totten! I've seen remnants of these shutters at countless American starforts, but have little evidence that they were ever successfully used for their intended purpose. So few of these forts were fought in the manner in which they were initially designed, with most cannon being fired en barbette, from atop a fort's walls...where there were no iron shutters emplaced.