I didn't realize we were talking MP. In MP that I know, people frown on buying cities, because it simply ruins the human war. It's the honor system rather than rules.txt, with great complexity regarding, may I bribe AI or not? Cities? Units? What if the AI bribed me, and so on.
For Single Player, bribing is the way to go. Cannon to capture the AI capital, then Diplomats from there. So many of the Masters have weighed in here, but I think I can add one point.
I go shopping with a Trireme and a Diplomat. I do not subvert an AI city because I have insufficient units to guard against revengeful acts. Instead, I bribe a unit, it becomes a None, then we hide in an empty AI fortress. Then bribe another one. Leonarno upgrades all of the AI Warriors and such. Eventually when my main thrust arrives, we have a merry band, already controlling all of the roads.