These are recently added bars to the windows of Ft. Pickens. I imagine they kept folks out during the time the Fort was condemned. Now they just make for an interesting contrast in this photo. Makes a nice monochrome shot as well.
Believe it or not, this is the site of a moat around the structure of Ft. Pickens. The water would have been in the green grass on the bottom of this shot; and would have been deeper than this ground is now.