I found out, at least on Prince, you really don't need many units. 5 units and a slight tech lead and you can win any war.
This is what happened.
I finished conquering my continent and went searching the world. Found America and Persia fighting and helped myself to some Washington that was just lying around defenseless. (5 units. 2 infantaries, 2 cavalaries and one cannon) I took it and Washington made peace with Darius, giving him all of america except one city. So now, what used to be a desert blue became a crowded red. I mean, really crowded. I had infantary and he had pikemen and knights; so my tech lead was quite strong. Still, he was sorrounding me completely, at least 15 units (and more coming) against my five.
Obviously, he declares war. So now I'm attacked everywhere and... lost nothing. Have some units on the yellow, others on the red. Use upgrades to heal everyone and oh man, what a massacre, he was just suiciding countless units into mine and not killing anything.
So, during his turn he suicides the pikemen into my infantry and on my turn, I kill the crossbow and trebuchet.
Those five heroes conquered half the continent. Sometimes I would say "this infantary goes to this city, this cavalry to this) and one unit was able to capture his cities.
So, yeah, maybe AI needs some tweaking, maybe my tech lead was just too big and I was supposed to be overcoming his greater numbers, what I do know is how satisfactory it is to see yourself in this situation and come out the winner. :lol