This is a thought exercise I've been wanting to try for quite some time.
Let's suppose for the sake of argument that we have a 17th level wizard. Let's further assume that he's an incredible cheapskate who hates spending money and hates spending experience points even more. (Let's also assume that the DM is sharp enough not to allow infinite wealth loops, since those take the fun out of the whole exercise.)
The challenge before us is this:
How cool a fortress can we build our skinflint without shelling out any money or experience points?
For starters, I think we could do worse than to rely on Stone TRap, from Shining South. This allows us to put massive blocks of stone into the air at pretty much any height we desire (as long as we cast it while flying near the target point.) Now, the ostensible purpose of the spell is to use these blocks of stone as a trap that will fall down upon a specified triggering condition, but if we make the triggering condition something impossible (the instant the planet turns into a large albino hamster named "Steve," for instance,) then the blocks are going to stay up there forever.
The blocks can support additional weight, as long as they're not the maximum size possible (which, barring CL boosts, is going to be 3400 pounds.) We'll make the blocks about 1000 pounds apiece, so each of them can support over a ton of additional weight. They're also invisible; we'll make use of that later.
So the foundation of our citadel is a core of these floating stone blocks. We'll then add a second layer of blocks around those (hey, they're free!) We'll then add a THIRD layer, and paint those so they can be seen. Each layer is large enough to individually hold up the whole structure; the outer layers block line of effect to the inner layers, and so render it extremely difficult to shut the whole thing down with MDJ.
Now, we can do some other fun things with these blocks. An additional layer of invisible blocks under the whole "island," and extending out beyond it for a considerable distance, can be given a different trigger; these become a defense against enemies stupid enough to mass under the fortress. Each block is going to dish out 20d6 damage when it falls, which will really mess up an attacking army's day.
So at this point we have a big floating island that has cost us nothing but time. What's next?