While worldbuiilder-testing the usefulness of UUs recently, I stumbled across something that made me slap my forehead and go 'why didn't I think of this before?'. Assume the following situation: you're Rome, there's a stack of Axemen approaching on flat terrain. Are you better off tackling them with Praeotrians or your own Axemen? It depends. Praetorians will lose to an equal-hammer force of axemen... the fights will favour the Praetorians until the fresh Axemen will mop up damaged Praetorians. However, if you have a comfortable local superiority in numbers, using Praetorians will have you lose less material than using axes yourself. Generally, cost-effectiveness will not matter as much if you have overwhelming local superiority. The poster child for this are Malinese Skirmishers: On flat terrain, they will beat any equal-hammer force of a single regular unit for a very very long time. They will achieve better than parity against anything until knights... but they will not allow one to overrun a weak opponent with hardly any losses unless you can get them to throw their stack away against your invincible cities/fortresses. Sometimes, cost-effectiveness breaks this rule though. Praetorians solidly beat crossbowmen in the absence of defensive modifiers. Not only in an equal-hammer-force (where the cheaper unit gets the advantage of pitting fresh against wounded troops), but infinite praetorians attacking crossbowmen will result in less loss of material than attacking with crossbows yourself. They lose horriby to Macemen though. Likewise, ancient-era archer-killers can still be effective in the medieval era provided there are no counter units around. Suiciding Quechuas, War Chariots or Immortals against Longbows to pave the way for cleanup troops may be more cost-effective than using Trebuchets. * The bottom line seems to be: If you can fight mostly favourable fights anyway, do so. If you can't be cheap and throw away whatever unit seems to be the best value for the hammers... THEN clean up with the best you have.