I think that the definition of early may make a difference -- at the outset, I think were looking at city defense of a warrior. Later, it may be a phalanx, or even two, maybe only a second warrior -- Ive seen two warriors make attacks out of the same city (and killing my horse in the process), but I have yet to see three do so.
My mind set for early warfare & conquest is contact & assault within the first five-ten techs or so. If the peaceful civs opt for a civilized development path, they may have nothing but warriors for defense -- so a horse could have a 50-50 chance of killing the defense (but would probably be too wounded to survive the counter attack). Similarly, an archer could attack & survive, take wounds, but be at risk of dieing in the fast build/counter attack as well. I figure most ais will have bronze working in their first five techs & upgrade their defense accordingly. (Hmmm this looks to be an ai cheat mode science & strategy cheat check agenda item.)
Once the ai has bronze working, the defense should be phalanxes, so the offense will need to be elephants, or at least vet chariots/archers. Even so, what is the number of troops for a good campaign -- will two units per city be enough? Or should the forces be increased to a level of three?