Does everyone remember the first historical battle on M2, the English versus the French?
What deflects the charge, besides the stakes, is before you even approach the enemy, a hail of armour piercing missile fire tears these horsemen to shreds. Granted, those were rather good longbowmen, but good crossbowmen or massed javelins can accomplish the same thing, or better; they usually have much more punch than an arrow, and crossbows in particular seem to be very, very dangerous to charge towards, providing they fire at the correct time.
EDIT: Of course, perhaps any missile troop works better en masse. Don't just use one or two... make a line of these guys and watch the missiles fly!
True, some of your late xbowmen might not be able to retreat behind the line in time, but if you spread crossbows lengthwise (helps them all aim directly anyway, no crossbows in the rear...) they can more easily disappear behind a line of spearmen, who take the rest of the charge. Then they can continue firing at a safe distance, providing your spearmen are in a deep formation which leaves gaps to fire into and helps them keep formation when charged.
Best advice of all:
Try a bunch of custom battles, spearmen and missile units versus a bunch of cavalry and infantry. Play around with the units until you get the hang of them, and learn to cause the most damage. Win the battle!
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