Wow really nice! I really like the desert setting, it is something that I personally wouldn't have thought of, but you made it work really well!
I also really like the use of colors on the piece! You depicted the desert really well and your shadows and lights are spot on!
My only critique though would be, that I don't particularly like the bottom dither. While your other dithers (on the top of the sky and on the shadows of the sand hills) work quite great, I feel like this one over does things a little bit. As it over lapses with the castle (losing a little the perception of 3D space) and I feel it doesn't really blend things that well.
So my advice would have been to maybe cut it down a little so that it doesn't go this high up, or if you decide you want it to go this high up, I would personally make it go, behind the castle and remove some of the dither around it (to show that the castle is on front of the sky)
Regardless, really nice piece! I can see that a lot of effort went into this! Good job!