Afforess
The White Wizard
I have a question about how fixed borders work. I claimed a tile outside one of my cities that my culture didn't reach yet (it was a hill across a river) and which had no culture on it. After signing an open borders treaty with the Hittites, one of their units (the Hittite Chariot) came along and claimed the tile for their city instead (though their culture hadn't reached the tile yet either). Why was the Hittite unit able to claim the tile even though no one had culture on it? Why did their claim overrule mine?
I need more details (screenshots maybe?) of your situation, because it is not possible to claim an already claimed tile. However, if you do not keep a unit on a claimed tile, it will be possible to be claimed by others.