It's a really cool game, heres how all of the classes upgrade.
So you've got a castle (with a fat princess inside) and a load of these pretty hat factories which produce class changing headwear that can be upgraded to give you an extra weapon/ability
The worker (the class that upgrades class factories and builds lots of other things) gets his very own bombs, which can be thrown at people across cliffs and whatever.
The warrior gets a spear/lance thing that has a slower attack and can rush at things.
The archer guy gets a shotgun which is great because he's always forgeted when fighting against warriors (or you can pusillanimous individual out and run away from them shooting arrows :D)
The mage gets ice attacks that freeze and slow down stuff
The priest gets the ability to turn into a necromancer (:O) he sucks health off of enemy units but he cant completely sap them
I wish this game had a stealth class :(