I'd randomise the castle structure and texture a bit. I don't know how. This way it's too regular. As to the horse, let's say ... it's a horse which returned without its rider, and then everything is okay.

No rain, it would change the mood. Perhaps maybe the contrast to make it more twards twilight or maby dawn?

look at the different scales of the bricks - a rider yes and maybe a guy at the top of the wall in chain mail shouting down at him. towers should be fatter. shorter? look at clouds.darker more menacing lightning? just random thoughts good fog, effective perspective, gate/gate wall masonry looks great.

What about a bit of vegetation in the foreground to add perspective and depth? Just a thought - like the misty feel to this!

a moat!

It looks like a wall and two towers, like theres nothing behind it... gives it a really odd look. Try adding something in the background, like a keep or something, I dunno. -Tempest-

If the rider is inside the castle you might want to tie the horse to something so he doesn't run off. LOL. Seriously, the size of the bricks on the right tower seem smaller than the rest. I also agree with the previous post. It looks like a castle shell. If it is empty and everything is destroyed you should put some damage on the walls and towers.