Well, there are some gamingly tricks that you can do.....
For instance, do not attack the settlement directly, but instead, park your army next to the settlement. Most of the time the AI will either attack you, or at the least will move some units next to your army.
Now you attack the army next to the settlement. Because of the "attack area" the units of the settlement will enter the field of battle as reinforsments. The trick is now that the goal is not only to defeat the enemy (which should be no problem as you are the attacker and preperation of your army is in full) but to destroy all. So, also capture all the routers. This way the settlement has no defenders left ond you can simple walk inside and claim it.
I know this is a bit of an exploit.........but in war and love.......
Ariaci