@ pix: The flat looking is caused by your flat highpoly...if you want nice depth in it you have to overdo theese parts in your HP. Hope this helps you a bit, cause this is just a fast NDo result.
@aNarchy The middle level (with the circle window) needs more detail than just the flat wall, struts/supports fancy gothic designs ect Also try adding a bell tower, where you'v just left it as flat top?