Actually the castle's perspective is way off. The horizon is typically always where your vanishing points go towards. Unless the castle was built at downward sloping angles, it's not correct.
The horizon line seems to be too bright/hazed and linear. The origin of light is obviously coming from the top left and the clouds darkern to the right. The sun can only cover so much surface of the earth before the horizon also give ways to darkness. Need more contrast and interest in the clouds and sky. You're trying to convey extreme depth with the landscape so the clouds should follow suite. Larger clouds towards the fron, smaller and flatter towards the back is the general rule of thumb
Also if you wanted to reposition the castle without transforming or distorting it, move the entire castle towards the bottom of the frame and it's going to be more believable. The composition is screaming for forground elements. I'll shutup.