There is definately a weakness in the AI when assaulting a city. One of the most obvious broken features is that the AI's units carrying ladders are able to set the ladders up but the AI never quite seems to send reinforcements up the ladders. Usually the reinforcements do a dance in front of the city walls while getting shot up.

Based on some posts, some problems are being addressed in the patch, namely the idiotic AI dance in front of the city walls. I am holding out hope that the patch will yet salvage this game. There are a lot of great features but the game overall is the weakest of the series.