I use time limits.

In M2TW I think they are a must because the AI will just stand there when it is the attacker sometimes, and I'm not going to come off my hill or from behind my walls.

The other thing I do which feels like a bit more of an exploit is win battles on the 3 minute city square rule. For example, if the AI has a small garrison but a lot of reinforcements coming I'll make a mad dash for the square and occupy it for 3 minutes before it can get the reinforcements brought up. I've also been known to blow the gate with cannon and run my army right past the guys on the walls to the city square.

As a side note, the city square clock seems a little buggy in M2TW. Sometimes I will get credit for occupying the square while there are still enemies in it.