Sun Tzu said that the greatest victories are those achieved without battle. But if you are going to fight a battle, the best way to destroy an enemy is to divide his army into smaller groups and destroy each one individually. This is defeat-in-detail.

In many historical battles, commanders tried to divide the enemy cavalry from the enemy infantry. Since most armies deployed with infantry in the centre and cavalry on the flanks, defeat of one or both of the enemy cavalry wings allowed envelopment of the infantry (as cavalry combat is a fast and furious affair that is over quickly, in contrast with the slow slogging match of the infantry). Off course, it did not always work this way as the victorious cavalry quite often got either caught up in the chase, or went of the plunder the enemy encampment..

However, most medieval armies were too ill-disciplined and unorganized to allow for fancy tactics. In the end, it all came down to whose cavalry prevailed. Provided the enemy infantry had already been thinned by the melee, the footsoldiers would be at the mercy of the heavy cavalry. That is why such importance was placed on knights: good heavy cavalry won the battles for you. However, it was this overreliance on heavy cavalry that almost lost the French the Hundred Years War. They assumed that, because they had more and better knights than the English, the battle would be a walk-over. The English, however, did not cooperate. Instead, they set a trap for the heavy cavalry, using the terrain and their massed archers to immobilize and thin them, and then finishing them off with infantry, including their own knights, who had dismounted.

The game fortunately allows much better control of your troops than Medieval generals had. It is much easier to split an enemy army, especially since the computer is not so good at keeping them together. Both light cavalry and missile weapons can be used to draw off the enemy cavalry flanks. Horse archers are ideal for this: they can stay out of range and still entice the computer into attacking with his heavy cavalry. They are fast enough to stay out of range and, once you lured their strong units far enough, you can either take on their isolated units or envelop their army. In the first case the goal is to lure the enemies powerful cavalry into a position where you outgun them, in the second case it is to make the computer send strong units (knights, mounted sergeants) to deal with a light unit (horse archers) so that your strong units can envelop his army.

So the tactics to remember are:
1) Dividing the enemy army
2) Envelopment of isolated parts