FAB does say 'slender' though - so I'm thinking less of a wider castle type tower and a bit more like a standalone church tower - they tend to be more slender, although square generally. Battle Honours do a nice slender church tower with an interesting, open, roof which might be quite good. Unfortunately, I'm on my phone and it doesn't like me using images! But if you go to Battle Honours 3d, search for church tower, you'll see what I mean.
If, however, square is not to your taste and you're wanting a round, slender tower - have you considered converting a lighthouse model into your wizards tower?