I play almost exclusively with the Aztecs and best I can tell, building up a strong defense and focusing on getting one step ahead of them technologically is your best bet. I know the few times I've been taken by suprise is while being way too focused on building units, falling behind in tech, and finding my warriors, archers, legions and horsemen suddenly facing off with Knight armies... The heal-after-victory ability doesn't do much good when you can't win a battle to save your life. : )