Hello!

The best results I've had for mass cavalry charges is to deploy my horse in 2 or 3 deep lines and surround the enemy army in a roughly circular or semi-circular formation.

Then group all the units to charge, then ungroup them (keeping them all selected) and then finally... double clicking an enemy unit that lies in the centre of the enemy formation.

All the horse then charges inwards in a fairly impressive and morale shattering manner.