Here's my tactic for assaulting an AI settlement with minimal casualties. My assault stack is:

1 general
1 ballista
1 spear militia
2 longbow archers or equivalent (no stakes needed)

Start by deploying all units far away from the gates. When you do this, the AI will deploy his units at the city center. After battle starts, inch the ballista forward toward the nearest gate until you can see the percent damage of the gate. At that point, stop and fire at the gate. At the same time, move all other units just behind the ballista. If done correctly, all of the AI units will stay at the city center and just wait for you to destroy the gate.

As soon as the gate is broken down, the AI will move his units toward that gate, but usually at a walk. Run your general into the settlement to block the AI units temporarily, while the infantry run up behind him. Put the spear militia in a blocking position on the main street, and pull the general back once the spearmen are in position. The archers move in behind the spear militia, either in the street or up on the walls, depending on the local geography, and fire past the spearmen into the enemy.

The kill ratio is usually close to 5:1 in my favor, and almost all of my casualties are the easy to replace spearmen. This makes settlement assaults so easy that it feels more like an exploit than a tactic. I wonder if there are any AI mods against which this doesn't work? I'm playing Kingdoms 1.5 with the Retrofit Mod, BTW.