I'm not so sure AI at this stage is able to analyse and evaluate the risk/success levels between:
- rushing troops through a single wall breach (which it used to do)
- finding alternate route: ladders, towers
- creating more than one breach in a wall
The basic human thinking normally goes - I can't just make one breach in a wall and rush it, enemy will definitely move in troops to protect it. I will breach/climb/use towers at several points - normally 2 or 3, depending on the size of the city and sizes of the defending and attacking forces. If the attacking force is too small to split, usually human player will not split it as the risk of routing will become even higher (due to being far from general, larger casualty proportion, etc.)
Not sure the AI is at this level yet. It does attack from several points in Shugun2, but it's not clear whether it calculates the number of sides from which to attack, its own size, the size of the defending garrison and the vulnerable parts of the castle which to attack. For all I know AI climbed walls of castles with open gates.
I'm not even talking about the use of special units. When have you seen AI use ninjas to sneak into castles or at least try to.