Diplomacy

Wait, you can buy a city from an AI? That never worked for me no matter how much I offered.

It's also the reason why I love the Huns. Get them friendly and just start stomping everyone on the map. As soon as you take a city, sell it to the Huns who will proceed to raze it for you. And be really quick about it. And be somewhat hated for it. While you just keep rolling and pass the angry faces hot potato to them
 
I personally can't think or remember anything you can trade besides the ones I wrote :)

EDIT: Defensive Pacts are completely useless. Never use them unless you want to elaborately put yourself at war with someone.

Anything with the potential for trolling in "friendly" games carries usefulness. I suppose the mechanic wasn't intended as a trolling tool though.
 
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