The outside of that very same opening from the previous picture. The rectangular opening above the gun port is a vent: Directly above the spot where each carronade was mounted for counterfire work was an opening, intended to vent smoke from the casemate and make it slightly possible for the guncrew to breathe. A brass grate was originally installed on the outside of this vent, to prevent whatever enemy soldiers were still alive and functioning in this valley of dismemberment from tossing a grenade or old sandwich into it.