For Verbose, I'm pretty sure that the Ballistae and that thing that shot fiery balls (I'm not good with terminology) were in fact field artillery and not siege weapons in the sense of knocking down walls. Besides that you missed what I meant about the cavalry, which is really my fault for not explaining. By all historical accounts Medieval Europeans bred for larger stronger horses, but by all historical accounts this was actually a ******** move. The Roman cavalry would be lighter and move faster, not as fast as a well bred Mongol horse, but I was thinking their goal would be to outmaneuver and pick off opponents like the Mongols did. I suppose I might be overestimating Roman ability with horses, but still, the main thing is you are overestimating the ability of Medieval armor to defend. They could stop arrows all right, but they were not strong enough to stop Mongol Composite Bows, so I see no reason to suggest they could stop a bolt from Roman Ballistae. Also I doubt that armor of any kind will help when a club bashes you in the face. It would help, but I think the main problem isn't the armor but the horse; if the Romans can't catch the knight, he could just run around chopping of Roman heads while Roman bows couldn't stop their armor. I think if it were Caesar he would just outfit a Roman spear with a hook and trip the horses, but maybe I'm overestimating Roman ability to adapt. I think if an educated modern individual were the ones in charge they would come up with many tricks the ancients wouldn't think of due to limited knowledge, but the Romans were excellent engineers. I think the Ballistae would be the deciding factor.