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_Fish_ Not exactly.
Class doesn't have anything to do with construction. Your build must look as if it has some means of support. So if you just had a floating castle with stonebrick on the bottom, it wouldn't work. If you just had a giant box fortress at the top of a 1x1x100 tower, it wouldnt work.
What supporting the floating build means, is it must look like it's suppose to be floating. This can be through...
Floating Islands with dirt looking like it was torn out of the ground to support the floating build
Magical methods, A giant lapis looking generation or crystal underneath that you can argue represents magic to support it,
Giant steampunk fans, Fumo did this in the past, not very RPG but very cool and original.
If it looks like it has means of support, its ok.