This is looking really nice really. You kind of worked yourself in a corner by making both the landscape and the clouds yellow. So I bumped the saturation up on the clouds and added a very slight shift towards orange. Also your horizon is lacking value to separate it from the clouds. They blend into each other, which we don't really want, so more dramatic shadows for them.
Did some slight fixes to the shadows on the buildings and experimented with some rim lighting, which I think looks fairly nice. Rays of sunlight will always fall in parallel lines, while artificial light rays will converge, meaning that most shadows in a natural setting will align with each other, while artificial lights will have shadows that tend to spread away and share converging lines.
I tweeked some colors, mostly dulling the blues down (yellow light on blue will make it grey out/ complementary colors). I also took some tiles out of your castles roof to add more age. Maybe just a slight hint of creeping vines might be nice too, depending on how long abandoned the place is.
I didn't do too much with your trees, since it doesn't really seem like you're finished with them yet.
I really like what you've done so far. I really want to see something in the landscape that will give us a sense of scale of the monolith out there. Maybe some old faded roads or ruins of a half buried city. Just something that will tell the viewer how gigantic that thing really is.
