im not much of a mattepainter but a few things i could recommend are :
1)the shadow it doesn't exactly line up and it looks to crisp i recommend tapering that off.
2) the castle doesn't have a clear fixed point that its attached to and clearly looks shopped which in mattepainting you dont want look at real pictures of castles on mountains for example this photo here - www.hotelclub.com/blog/wp-cont…
Notice how the cliff merges with the castle in it looks more realistic than if it was just plopped on the mountain i recommend doing studies of castles and mountains.
the good- almost everything else looks great to me the clouds lighting and mountains ect.
please dont take this feedback to harsh id love to see what comes next