[Wauthan]

I'm not sure what role admin playes in supply distribution but I know you get a supply boost while inside a four province radius of a fortress. High Growth helps a lot, but only in your own dominion. A good castle helps anywhere and since Caelum can and should launch attacks behind the frontline this might be a better idea than improved scales. All in all I'd say you made a good decision choosing the Wizard's Tower (though it's a heavy investment).