Citadels?

[sunsmountain]

In RTW, despite the grand walls and large cities, the AI regularly only had one or two units defending... which lead to incredibly boring and un-epic siege battles.

If you attacked the AI after the AI attacked you yes, cities will be mostly empty as their armies search for you. But if you're the first to attack the AI, you can catch them defending. The Greek city states are notorious for epic siege battles, perhaps you should leave them for last (i know, they're juicy).