Consider this a guide. Kind of cheesing though. What I do is press F7 to call everyone to retreat, and immediately after that call everyone to stand at a point not too far or too close to the enemy. You need to do this several rounds until your soldiers stop rushing to the ladders. After everyone behave themselves, I will go to the barricades and start shooting the enemy archers. Do not let all your men retreat, leave some of them at a reasonable distance so that the enemy archers would have some difficult targets otherwise they will focus on you. After you deal with the enemy archers, set up the ladders. Enemy infantry will come to disable them one by one, and you can just shoot them one by one. Easy. You may need to leave the battle and reenter several times. Using this method, and you will conquer every minor settlement on the map with minimal casualties.
For large cities that have more 500 garrison and militia, you need a reasonably small elite force, so that they will sally out and fight you on the battlefield. That would be much easier if you know how to deal with their archers which usually comprised half of their forces. The easy way is like this: you charge to the enemy force which is definitely moving towards you, and focus on the archers. They will not shoot you when you are close. Instead, they will move around you. So you could simply slash them down easy, it's like hunting bisons. After your archers and cavs deal with their infantry, your cavs can join your hunt. You would need to leave the battle and reenter if your force cannot take out the enemy infantry in one go.