I've been wondering... given that IRL the Celtic chariot was really more of a "battle taxi" for a senior warrior, and the crews wear about Bataroas-level war gear, wouldn't it more or less work to simply give the unit secondary defense stats more or less identical to the primaries, to represent the fighting crewman fending off enemies ? Granted that doesn't really account for the way the horses are sorta exposed save for there not being a shield value in the secondary line, but since you kinda need to abstract anyway...