In my local mod I have made chariots a some different than "normal" citizen cavalry. They do not upgrade ranks, you must purchase improvements called Medium Chariot (City Phase) and Heavy Chariot (Imperial Phase) in order to make them uber good. It is my hope that I can change them further when trample damage is implemented by developer and improvements to the visiblegarrison part of the xmls that can make the archers passive garrisoned units and the chariot itself is used like a ram to smash into enemey units (trample and charge, you do still plan these right?). Like someone else said, the chariots would almost be a suicide unit (think of Petards in AOE2, but used against units instead of buildings), and the archers are just passive guys shooting at nearby enemies. I wish to do the same with elephants (but obviously less suicide and just more chaotic and crazy, for a big price).