This is one of the three smaller sentry boxes at the Castillo de San Marcos in St Augustine, Florida. The walkway is extremely narrow and a soldier armed with ammo and rifle to get to the box would have been an interesting sight.

Still, it was surprisingly well-sheltered. The coquina-shell block walls were highly resistant to cannon ball damage and the small openings would have been difficult to shoot into from the water.

Again, I've taken the liberty of editing the background slightly to keep the focus on the sentry box itself.