My method is to place mercenaries and weak units at the front of the siege with battering rams (decoy), while my elite units (archers and heavy infantry) are at the sides with ladders.

I start by sending up a battering ram to capture the enemy's attention and to make them move toward this area (by default the enemy's concentration is heaviest at the place where you start in battle mode). Once the enemy is distracted I send my elite units to the walls with ladders (unless the walls are heavily defended). The advantage of ladders are that units can run while holding them, so they can quickly get to the walls and kill the few units defending them.

While the enemy is concentrating on the ram, I've captured the walls gates. Then I send my elite units towards the enemy's forces and get my archers onto the walls. Since the walls have been taken and their soldiers are under attack the enemy ususally pulls back to the center, allowing me to capture the front gate and bring in my mercenaries and weak units (I never let them break the gate because the enemy would just swarm them).

Now that my mercenaries and weak units are in the castle I use them as targets to make the enemy fight me, while I pelt the enemy with arrows. If the enemy has ranged units I just plow my mercenaries and weak units in the enemy so that they have to fight me. If these units fail (they can do against the general's bodyguard) I just use my heavy infantry to mop up the remaining soldiers.

While I may suffer high casulaties most of these are from low cost units are expensive mercenaries.