Unlike many games where a unit of a type is a unit, in this game experience counts. Leaders can train fighting men to improve their skills. A basic soldier has a ten combat rating. A seasoned veteran 100. And the difference is indeed noticeable.
More experienced units gain special attacks and can sustain more damage as well.
Thus cranking out units is not ultimately as effective a strategy in this game as in others, not only because you must recruit one of your peasants to become a soldier, and they are limited in number. Rather, building a core fighting force augmented by high tech weapons and a spy corps is the best path to victory. Unfortunately, the kingdoms in the game don't really provide different playing styles. Each is graphically distinctive, but while units are different, mixed unit armies are always the way to go. And while each kingdom can summon a different higher power, they don't have that big an impact on the game.