Hi Hieu... I would suggest working on giving your environment a direct light source. Right now you just have flat value. Maybe try to find some images with a kind of lighting that you like.
Hi Hieu, this may sound cliche, but I think the castle could be scarier. Maybe the trees in the middle ground could be scarier as well, so theres a kind of transition, like we're looking through J&J's eyes from the good side of the forest towards the evil castle. Just my 2 cents.