I agree with what's been said so far; it does have a very messy style that I think would benefit from some cleaning up. I'm not a big fan of what seems like very random placement of dithering(just random spotting on the rocks, for example). It make it look like a color reduction or something -- this is more readily apparent on the stairs. Cleaning that up and replacing it with either a smoother coloring or a more structured dithering will go so far. Same can be said for the AA in the piece, for example on the dune already mentioned. While on the subject of the dune, I find while the technique is solid, the shape of the dune is very unnatural. It's so short in length, but so high. As well as the height doesn't seem to follow any sort of natural shaping(the ends fading into the sand). It doesn't look like something you'd find naturally formed in the desert. While making it longer will probably not be a possibility, you can make it look more natural by making it maybe half the height and maybe make the shape a little less dramatic.