In the larger castles you cannot move the battering ram past the gates to take out the inner keep gate. You must punch a hole in the wall with well placed catapults and then move the RAM in. When in a siege, quickly move the troops placed by the computer out of arrow range if you want them to survive. In a foot battle, remember the strengths and weaknesses of troops. Pikemen can stop horses easily.
Swordsmen move slowly while mace men run quickly.
Bow and crossbow men must be protected if they are yours and killed quickly if they are the enemy. Knights or mace men can out maneuver the foot soldiers and get to the arrow tossers much more quickly. Patience is very important. Wear down the defenders until you have tactical and numeric superiority. Move fast to destry archers, if you can get to them with melee troops.