Since the wall was built sloped, there is no valid reference in it. I´d use Peter´s advice but with great care. In general I believe the pic is almost level. Maybe some 0,1 or 0,2º CCW will do. Or maybe not. Try it out and if you want post the results here.
I´m a bit more worried, however, about the softness of the image in some parts, specially the blurriness on the right wing. Also be careful with the blown-out whites on the wing edges, for example.
Hope it helps.
"Faithless is he that says farewell when the road darkens." J.R.R. Tolkien