Here goes nothing...
I'm going to focus on the castle in the sky one
The chain...I have a feeling the chain looks like it would not hold a larger animal... Shouldn't it be much larger when its near the viewer? And it would be far more "epic" if a chains peace is at least a means hight... I know it could be a magic material, but...then other chains shouldn't be visible in the distance.
The Castle itself looks like it's semi transparent. Especially the right-most "sky-island", the left-most tower, and the top of the "central tower", blends with the clouds in my opinion. I never really was good in finding things in the work's of others, so I guess those are not mistakes, but rather things I would do different :P