Spears or pikes to hold cavalry charges and gates - they are the center of your infantry line. Heavy infantry (dism. knights) to flank, spew from the back lines or hold walls. They are deployed on both "ends" of your infantry line. Keep the spears/pikes in guard mode and circle the heavy infantry around so they strike at the side of the enemy mass of troops. Charge your cavalry from the rear.

This happens easily in Vanilla M2:TW since the battle AI is so horribly inept. Try installing the Stainless Steel mod to get more challenging battles (and more historically accurate rosters for the various factions)