Couple more ways...

You can raise a terrain column and pull iron pole or similar support off it.

You can attach a support pole to a tree, it is either ground or ground + 1, I forget. This has some risk as trees near your home are vulnerable to raids and losing support = crash and burn, so if you do it, protect the tree.

wood can go on top of red stone, at least 2 more floors.

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putting everything together, I sort of have a 'design' that I use frequently.
that is to dig out or build up the first 'level' of the home only using terrain raise/lower. The top of this should be perfectly flat and all the same height, making a ring or rectangle empty space that forms a 'room'. Then come across the top of this with your wood iron poles and put down a stone floor. you can put in walls and floors and such below or not, but that is cosmetic only, it won't support much. If your build is very large, a central raised column of terrain should be placed as well to offer support up top later.
then build your castle on top of this, with stairs down. This gets you an entire level for 'free', possibly 2 levels if you don't mind low ceilings. Then you can put AT LEAST 2 floors of stone and 2 floors of wood above that. The total is at least 5 levels, 7 or 8 is probably doable with extreme management of the support system and that mentioned middle terrain column. I like higher ceilings more than additional levels, though.