[Agema]

Cavalry could be used as shock troops prior to the stirrup, just not against decent heavy infantry. In a battlefield situation, calvary generally would not slog it out with infantry, they'd use their manoeuverability to withdraw and charge again. That's getting way beyond the scope of Dom3.

Secondly, it's not really the professional soldier that doomed the era of knights, it's just the firepower. There's no point training and equipping a knight who can be downed easily by a poorly-trained peasant with a powerful projectile weapon, who you can't get at because another load of peasants are holding long pointy sticks. Professionalism hasn't that much to do with it - professionals existed all throughout the medieval era to varying degree in varying nations. Byzantium, for instance, maintained a standing army just as the Romans had before them.