Re: Use a sword
I recall knights in armor sort of fell out of favor when armor-piercing ranged weaponry became all the rage (longbows and crossbows and finally firearms). Forget up close and personal; reach out and touch someone is the preferred doctrine now; the farther out you can hit the enemy, the sooner you can finish the fight (since to get in melee range, you have to MOVE into melee range--by that point you'll have been raked several times over). As for the horses, consider that horses can run faster and provide more force than a man can; if YOU don't bring the horse into play, THEY will to their advantage.
The final rule of war: when things get ugly, the rules go out the window.