Bearing in mind that I don't really recall any specifics at this point (it's been a month or two since I played), my opinion of the AI in Medieval 2 is that while it's somewhat better at assaulting cities than in Rome, it's still not what I would define as truly competent.
There's less aimless milling about of troops and the computer is more decisive in its actions, but it doesn't utilize its artillery that well and sometimes employs units ill-suited to attacking fortifications. (I do remember defending one city where the emeny attempted to storm over the walls with some crossbowmen, even though they had a number of swordsmen and spearmen at their disposal.)
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