Each swinging door had a lock like this one at the bottom, which kept it shut until the gun crew was ready to fire. These spring-loaded latches were undone and the gun was fired at the closed doors. The heavy doors were balanced so precisely that the gases emanating from the gun barrel ahead of the projectile pushed the doors open, and then they swung back shut and relatched themselves as the crew reloaded, safe from return fire.

You will note, however, how clean and precise the shutter assembly is, as opposed to the iron embrasure to which they are attached? You guessed it, long before I did: These shutters are reproductions.