Or maybe I should have asked "WAS this your cat?" Each of Fort Gaines' 5 bastions has a small powder magazine built into the corner opposite the bastion entrance. If I hadn't taken a spotlight into the fort on that trip I probably wouldn't have seen the cat or realized what it was. At first blink it looked like a plastic grocery bag.
There are plenty of places in a fort for curious cats to explore and hide, but Felix might have run across one of the resident foxes. If the fox ran him in here, the only way out is back the way he came.
Obligatory Fort Content: the rectangular openings to the left and above Felix are part of the ventilation system. Each one does a zig-zag through the wall, allowing air to pass in and out but keeping any sparks, embers, or cats from entering the magazine.
In the upper opening, you can see a brass or bronze grate that adds another obstacle to sparks. This opening vents to the outside of the bastion; the other vents into the main room.
You have to stand in the room to appreciate the amount of air that's flowing through here without any mechanical assistance. That's by design: keeping the air moving around helps to keep the gunpowder dry.