This is the interior of Fort Gaines' Bastion D. This pic doesn't do the view any justice, but it's the only one I could find. Time for another road trip!
To left and right are paired gun emplacements, each separated by a thick pier, each with an arched ceiling. Those four "side" arches intersect beautifully with the main one.
The door leads into the bastion's small powder magazine. The "skylight" is part of the ventilation system that kept air moving in the magazine to keep the gunpowder dry.
That big manhole-looking thing on the floor is indeed a manhole. Rainwater was gathered on the roof, ran down through the piers, and into the drain system under the floors. I don't know whether it's a cistern or just drainage; haven't gotten my hands on any plans that show the foundation structure yet.