It's very pretty! The main thing I think you have to work on is the ground... right now, it has the same color scheme (palette line?), brightness, and apparent size as the buildings in the background. It's more of a problem in the bottom area of the second screenshot where you don't have the rocky gray floor providing a clear boundary. You might want to soften the contrast on the buildings a bit, but either way, the ground needs a lot more structure and variety. Here's a mockup of one possibility:

Obviously you'd want to choose much better colors, that's just the general idea. Also you have a lot of room to play with holes in walls with inventive shapes, perhaps even with the edges receding into the background. Whatever the choice, though, variety is necessary.


ninja'd by mike. I do feel, though, that Palmtree Panic is a better reference than Green Hill Zone, since it uses the same pattern.