ok I'm going to say something I believe some people that posted above me will agree with.
if an object is away from the viewer it will have more atmospherical density (most of the cases it will be more blue) wich you have cleverly added to your scene. looks nice!
but another thing that also happens is that most of the details get lost, because well, one: you are too far away to see and two: because the atmosphere density sort of reduces de contraste thus killing some of the detail information.
right now, the tree that you have in the first level of the background has detail that is pushing it forward to the playing level.
Im not going to say that is is not obvious that the tree is in the background, but it definetly would be better in my oppinion if it was less noisy.
oh and btw, the cast shadows on the columns arent really like that. they arch up in the midle, and down in the edges of the columns.