A mysterious, small alcove along the front of the terreplein on the water-facing side. Big enough to hold what, exactly? In some cases I've seen little hideyholes built into gun positions, in which gun crews might huddle whilst under fire (get out there and serve your gun, you slovenly soldiers!), but this certainly doesn't seem big enough to hide in. Maybe this is where the gun crews could stick their expensive boots for protection during a bombardment? Or could it be a vent through which gunsmoke from the casemates might be ejected? Surely that smoke wouldn't be directed up at the gun positions on the terreplein, which is where this is.