I don't know if this at all aplies to MTW2 but it does to RTW.

Just have your diplomat give the city as a gift away to someone you are at war with, then invade the city which lacks a garrison or governor and after no opposition you can enslave the city (taking on 50% of the population) or if you are not feeling chivalrous massacre them and get a great economic bonus.

It doesn't work in Barbarian Invasion so might have been done away with in MTW2.