If you have to take down a fortress you might need a big army. Also I dont see why anyone would want to hire someone else. First of all, if they actually do have enough money to hire someone why dont they just attack themselves with their own money. If they have the money to hire an army, im sure theyll have the money to pay for their casaulties. Also, it would make them look bad in front of the enemy.
I wish there was some kind of an assassin feature though... Although there's noone to assassinate. Wait, maybe heroes? Although it depends on how hard it is to make them.