A nice BG, and a cool style.Ã,Â
What bugs me:
-judging from the tower, the lightsource is located at the right source of the picture, but if you look at some of the rocks, you can see that some shadows are at the right side when they should be on the left
-IMO, the clouds don't fit in the BG very well, try using cell shading on them like you used on the rocks
-if it was night time, you could color windows yellow like there's a lightsource inside the lighthouse, but in daytime the windows would reflect the sky color
-no matter how the perpective is warped, the horizont line should be, umm... horizontal (where sea meets the sky)
-you could add some simple texturing, just a few lines here and there so it doesn't interfere with your style
-the shaded color of the tower blends in with the sky (on my monitor at least)
That's all.
EDIT:
Here's a quick edit:

What bugs me:
-judging from the tower, the lightsource is located at the right source of the picture, but if you look at some of the rocks, you can see that some shadows are at the right side when they should be on the left
-IMO, the clouds don't fit in the BG very well, try using cell shading on them like you used on the rocks
-if it was night time, you could color windows yellow like there's a lightsource inside the lighthouse, but in daytime the windows would reflect the sky color
-no matter how the perpective is warped, the horizont line should be, umm... horizontal (where sea meets the sky)
-you could add some simple texturing, just a few lines here and there so it doesn't interfere with your style
-the shaded color of the tower blends in with the sky (on my monitor at least)
That's all.
EDIT:
Here's a quick edit: