to be your first matte p. it’s pretty cool !
here are some pointers:
the main focal node is your castle right? cause the eagle is fighting for it, it’s too strong for the eye, I think maybe it should be smaller and with motion blur on it,
overall the image needs a photoreal exposure, the castle shadow side should be darker as well as the shadow side in any other element,
the rocks/cliffs are too color saturated for an overcast day
you have to work on the detail where the grass meets the castle, it’s too soften and blury now
also think about real life elements, there must be a way to get to the castle, a road, maybe a fence as well, not just an structure with no elements around it , just perfect grass?, all those little details will help you sell the image
also the river is too bright in some areas where should be darker, the river’s highlight shoul be only close to the top
always look at real photo reference before starting to work , stare for minutes to the photo and see what makes it photoreal, and just by studying it you will learn a lot