Sorry for selfishly asking this...
Poland's forts cause culture bombs... can you build one, then remove it? If so, does removing it remove the 'bomb'?
That has to be tested, of course, but I don't think there's a way for a city to lose a tile once it's been included, except for swapping it to another city or a culture bomb on the opposite side (also, what happens if Poland and Australia are neighbors, and one builds a fort to grab some tiles, while another one builds a pasture on the other side of the same tile? Later one wins as with fortresses in Civ 5?)...