one set of civilizations is fighting another one and trying to capture a city. This will take several turns due to stackless combat. (this happens quite frequently)
culture grows extremely slowly + melee units move 2 tiles = you can hang around on the border of the city that's about to fall. if the culture radius is large, you can have an open borders agreement and declare war without getting kicked out of the borders.
in either case, just wait for the city to have low HP and then move in with your 1 or 2 warriors/spearmans whatever
pretty much a free city and you don't even need to defend it too much because the civ you took it from is already weak and still fighting the other AIs...

culture grows extremely slowly + melee units move 2 tiles = you can hang around on the border of the city that's about to fall. if the culture radius is large, you can have an open borders agreement and declare war without getting kicked out of the borders.
in either case, just wait for the city to have low HP and then move in with your 1 or 2 warriors/spearmans whatever
pretty much a free city and you don't even need to defend it too much because the civ you took it from is already weak and still fighting the other AIs...
