My entry for this week's #architecturalphotography contest sponsored by 
As you can see, there are 3 different sets of arches going on here, all at angles to each other, and all working together to hold the walls upright against the massive pressure of all the sand which composes the parade ground overhead. The uniqueness of this fort is in how the inner retaining walls of this gallery around the perimeter were constructed. Instead of being solid walls all the way up, they were designed to be half walls that allowed the sand overhead to run down and come to a stop at its natural "angle of repose" (not captured in the picture.) Thus the sand itself helps hold up the fort. To prevent the walls from buckling outward over the years, they were not built straight, but were instead designed as arches lying on their sides, bowing inward against the earth. At the time, these features were considered to be a major (and innovative!) departure from convention.
On the right (and out of sight of the camera) is the outer gallery wall, in which are narrow rifle loopholes. In the winter, at the right times of day, the angle of the sun is just right to concentrate the sunlight in blazing spots on the opposite arches, as opposed to the diffused light you'll see in most photos if you do a google image search on Fort Barrancas.
Fun facts:
This fort was built with 6 million locally sourced bricks. Does that sound like a lot?
Well, Fort Pickens, across the bay, was built with 21 million bricks. Needless to say, building the forts was good for the local economy!
During the Civil War, Union troops occupied Fort Pickens. The Confederates (under Braxton Bragg) held Fort Barrancas as well as Fort McRee, a third fort that completed the triangle of defenses guarding the entrance to Pensacola Bay and the navy yard inside it. Following a minor Confederate land attack on Fort Pickens, the Union forces decided to redeem their honor and get revenge by rolling out the cannons and duking it out with the other forts. The two day bombardment had little effect on Barrancas because of its distance across the bay and height on the bluff, plus Bragg had taken the precaution of piling extra sand around the seaward walls. Fort McRee was unfortunately right at hand and completely exposed, being out on the opposite barrier island to Pickens; so it took the brunt and was destroyed. Over time the remains vanished from sight as the channel into the bay shifted west, and if not for historical record, you'd never know a third fort had existed.
Hope you enjoyed this little bit of history! If you're ever in Pensacola, stop by this cool little fort and check it out!