A:yeah i know its unhistorical but COMON aztecs attacking europe? thats not unhistorical, thats unrealistic. But the chinese had a fleet and army to conquer europe, the chinese emperor just destroyed it a bit before the game starts (lol). Just make it that china is a playable faction.. they had problems with the mongols too so why not play a game fighting back the mongols (as chinese). Probably chinese win and go for europe next? And if you activate the expansion it shouldnt make them invade you always, there should just be a trigger that they actually fight mongols back or sth. For 40-50% the game will be like always, but for 20% or so the chinese will run over all europe (instead of mongols) and for 30 probably they will beat mongols but do nothing (so there wont even be mongol invasion). But cause there is china on the map, the player actually has the chance to go other way around (invade china). That might be ahistorical but not unrealistic.. imagine you own entire europe and are the ultimate imperator (like alexander the great), why shouldnt you just turn east then and try to make alexander the great´s dream happen?
