Originally there was no intention of mounting guns in this gallery, but as the work progressed the fumes from repeated blasting almost suffocated the miners, so it was decided to open a vent to let air into the tunnel. Almost at once it was realised what an excellent embrasure this would make for a gun, so one was mounted without waiting to reach the 'Notch'.

http://www.the-rock-of-gibraltar.com/Great-Siege-Tunnels/