Quote Originally Posted by Minamoto Yoritomo
I ran into an interesting phenomenon early in my last campaign. My initial arrow barrage reduced the attacker's heavy cavalry down to one horse, so I focused my fire on the enemy spearmen, knowing that part of the threat was neutralized. The computer moved his single horse back and forth near my archers' flank until they routed, something that completely surprised me given the fact that I was winning the battle handily (and still won handily, despite my cowardly band of archers running away from one guy on a horse).

Anyway, the moral (no pun intended) of this story is to deal even with insignificant threats on your units' flanks in order to keep them from routing.
Yeah, the computer treats 1 man the same as 60 men for morale purposes. A bit daft really. They fixed it in Medieval.