From the vantage point of the barbette atop Fort Knox's caponierre, it's Battery A! This battery faces the direction from which it seemed most likely that an enemy might come: Straight up the Penobscot from the ocean. Also, we see the Penobscot Narrows Bridge, atop whose right-hand support is the observatory we will later visit!

Fort Knox's Batteries A & B were the first fortificational things built at this location, so naturally by the time they got around to actually arming these batteries all of their gun mounts were wrong for Rodmans, and had to be reconfigured during the Civil War.