Fantasy art piece struggle

This is an artwork ive made of a mountain scene done digitally, ive been struggling to add a fantasy element i didnt really plan out the fantasy element well, i wanted the piece to have a castle and a dragon as the fantasy element i finished the mountain first without including either in my rough sketch. I finished the rough sketch of the castle but im struggling to add the same level of detail as the mountains to it and make it the highlight of my composition,looking at my piece i feel it ruins the piece and im better off without it But i still want that fanstasy element and want the final work to have both the castle and the dragon. Any idea,tips etc on how to acheive it? Aside from the castle how is the overall composition,colours,values etcs?