An unoccupied casemate. I was curious enough about the slots under the openings to ask Paul Branch, the resident expert at Fort Macon in North Carolina, about them after my visit there. Here's his learned description:

The chassis of the gun carriage would have extended into this triangular opening under each window and locked into place with an iron pin that was dropped down through a hole in the brick in the bottom of each window that aligned with an iron ring on the front end of the gun carriage chassis.