Ft. Nelson - Inside North Caponier
This is the inside of the northern caponier, which, as you can see here, contains two stories of artillery and infantry firing ports.
For enfilade fire along the ditch, canister shot would be ideal.
I believe that this is a 32 pounder naval rifle, modified from muzzle loading to breechloading, which actually fired a 56 pound projectile. Lateral aim was assisted by rails, and an incline helped with the recoil.
Infantryman's view along the ditch through a loophole.