We had a challenge once and someone did a floating castle where it was sharp but the background was all blurred and it worked a charm. I recall a chap called Torq did a battlement map with various layers to it all blurred at different amounts and it looked great too. So thats one way to cover it. Also, you can do clouds under the castle. Tho they do have to look like real clouds in order for the effect to work else it looks like fog.