Don't think I've seen that one with the man sitting there, thanks for posting.
Here's another view a little to the right of where that photo was taken.
Any attacker would have a hard time assaulting that, especially with artillery pieces placed behind the embrasures in the parapet. Notice how in the photo above the cannon would be positioned to enfilade or fire horizontally down the line of where the photo was taken. Any approach to the fort could be swept with canister. The fraises (sharpened stakes), abatis and the ditches are constructed to slow down the attacker.
What I find most interesting about this, and other photos of Civil War sites, is the lack of trees compared with these same locations today. I guess we figure out from photos like this where they all went.
Here's a map of Fort Sedgwick. I believe the above two period photos were taken along where the "Main Battery Front" is located on the map. I've marked where I think they were taken from and in which direction they are facing.