"But what of the Castle in the Air?" the bug objected, not very pleased with the arrangement."Let it drift away," said Rhyme."And good riddance," added Reason, "for no matter how beautiful it seems, it's still nothing but a prison." (18.76-78)
The Humbug just sees a beautiful castle. It's up to Rhyme and Reason to remind the others that the place's beauty doesn't change what it really is: a jail. Remember Alec's advice? Keep your eyes (and your mind) open. Then you can see things for what they truly are.