Windows are usually inset, not sticking out from the structure. Switch the light and dark, and that'll fix it. Why are there only big windows on the front, and small windows on the side? I'd mix it up a little, so it looked like they're all part of one building. Something about the bottom of the elevator shaft looks jumbled. I don't think the shadows are right. The wall should cast a shadow on the bottom of the shaft, and the elevator should cast a shadow somewhere. There's a part in the back bottom corner where the windows line up with the windows on the wall in front, but not with the wall it's on. The left side is being held up by pillars, but not the right. It looks a little unstable. Speaking of pillars, how is it flush with the front on the bottom, but not on the upper floors? Somehow your perspective got skewed. You should probably keep the spacing of the pillars even as well. The front one is much farther forward. Some of your shadows are only being cast from the front of the object, mostly roof elements, the back should be casting a shadow too. Shadows don't usually layer unless there are multiple light sources. If it's already in shadow, it won't cast one. I like the colors, keep at it.