I'll start with the head since you've mentioned how difficult it was to do. Personally I think it looks great and you've captured the side view of it perfectly including its anatomy. The facial expression goes very nicely with that side view too.
As for other aspects of the picture, the colors look very good as well as the character's overall design and lineart. I like how the eye came out.
I want to critique the shading, but first, I'd like more clarification on it because it's a bit hard for me to pick out which areas were shaded against the brown color. Once I get that clarification, I can give a better response.