Hmmmm...if that were taken through to it's logical conclusion, an upgrade to a castle with fiddly bits should be cheaper and quicker than an upgrade to a fiddly bit free fortification, shouldn't it?
Quite agree, but you still pay the same for a castle whether you're making the leap straight from keep, or keep + curtain wall + ballista towers.

Another bit that bugs me is that when you are on a razzia and trying to raze an enemy province you've captured to make some cash back, when you destroy a castle that has "fiddly bits", all you recoup is the cost of the last upgrade. ie destroy a basic castle you can recoup (iirc) 1000fl, but destroy a castle + upgrade, you only get 200fl or so. This is just wrong. Not that I destroy castles much - it's usually a desparate attempt against a powerful enemy when I have no chance of holding a province from them, or part of a Pope-on-a-rope set up.

I still consider 'upgrades' to be a more expensive way of developing a castle more slowly.