Bastion A, Fort Warren's eastern bastion, was roofed over in the 1870' was originally similar to the fort's northern and western bastions, which have casemates all along the edges, but an open space in the middle. This roofing-over was enacted in order to mount 15-inch Rodman Guns on the bastion's barbette tier, but it created a huge new indoor area, part of which became known as the Dark Arch. I thought I was looking at the Dark Arch here, but this is just the Dark Arches' vestibule!