I learned this the very hard and painful way. If you and an enemy both have open borders with someone, you can both move into that civ's cities, which will cause combat. This applies even to situations you wouldn't expect - land, naval, and air units all enter some form of combat. For some reason, I thought this wouldn't happen to a massive squadron of fighters who I was going to use to bombard an enemy fleet... that fleet waltzed into the city and destroyed all of my fighters. The order of supremacy goes something like this, I think... for being in a city, 1. Land 2. Sea 3. Air I.e. land will always destroy sea and air units, no contest, and sea units will always destroy air units, no contest. Can anyone confirm/deny this? It would also make sense that the city's defense values would not have any effect on the defenders in a land-land combat (besides terrain), and would act as a coastal square in a sea-sea combat.