I've recently discovered that you can win most any province held by no more than a thousand troops simply by sending in your king, and no one else, over and over again until you win the province. see, your king will regenerate a portion of his security detail after each battle. eventually you'll have the enemy whitled down to a size that the king can beat. do this once, and you'll get some good virtues ("leads from the front", and such), do it twice and your king will be thought a fool and a madman.

at anyrate, what used to work really well for me in shogun, and still works, but not as well, in medieval is that any force that if you have reached the troop limit on the field, your enemy, no matter his size, can be defeated. my troop limit is somewhere just over a thousand depending on unit type cause my computer is slow and I have adjusted unit sizes to cope. but if I go against a foe who fields ten times that many troops in the battle, it doesn't matter, so long as I force him into "perma-rout" so that his reinforcements all turn back once they come on the scene. you must do two things to ahieve this effect: 1) kill the general, or at least make him run away. 2) take the real estate that the enemies reinforcements will enter on. you should also split up the enemies archers (and any other units that are weak in small numbers), and reduce them as much as possible without causing them to run away, thus denying, or at least delaying, further reinforcements. now in shogun once you've achieved this you will see men enter the field, and imediately turn back, and that will go on for some time, but in medieval they have made it so that the archers can attack from behind the border area that you units can't cross. this is somewhat annoying, since your archers cannot return fire. but if the reinforcements are spaces enough apart from eachother, it should conclude in your favor quickly.