it looks like your light source (sun) is coming from sort of straight ahead/off to the right of the camera... i would expect the tower and the doorway building to give off more shadows from that direction. if you're going to keep that sun, i would think they would be pretty sharp contrasty shadows. that and adding some of the smog that mhcannon was mentioning would definitely pull the scene together more and make them not seem like they were just layered on top. it's looking good though, keep it up..