I just noticed a potentially game-breaking bug / exploit related to building an outpost in an allied settlement. I looked around a bit and didn't see anything related to this so apologies if this is old news.
Upon obtaining an alliance or if you haven't built an outpost in an allied city yet:
If you build and immediately demolish an outpost on the same turn you get your money back and a +10 diplomacy boost with any nations at war with the ally. You can do this over ad infinitum, and while it does mean you will get a cumulative -10 hit to your diplo with the ally as well, if it's an ally you don't care about...