[Suggestion] City Base Defence?

I have so far taken an enemy city within the first 50 turns, every game I have played. Basically killing half the enemies in the game almost immediately because the ai ais unable to defend them at all. Against my sovereign and an adventure with combat rating of around 20 they have defend with a lone merchant or equivallent. I think that to stop things like this, the citizens should arm themselves and defend their new homes or maybe the little palisade gives the defender a large bonus, or something that makes it harder to take cities. I am playing normal difficulty but normal should not be helpless. Anyone else thinking the same?