There is a small amount of keystoning because of the angle of view and use of the 90mm lens; however, that results in only a slight exaggeration of the true shape of the front of the building. It is actually more narrow at the top than at the bottom.
That's funny, I didn't even think about the angle of view/camera back tilted as being wrong...I just accepted it as a compositional choice. It's fun to break rules! I like the strength of this one.
Thanks for the reply, Ria. I was really wondering if it was intentional or an accident de parcours. The Greek knew how not to build structures that optically didn't lean backwards ;-)