I've received a PM regarding a game that froze when the chokepoint defenders were demanded to be removed due to culture from a nearby AI city going over the space. If this happens to you, I (or another staff member) may be able to modify the save file to permit you to continue your game. Please PM me if you have any issues like this in your game, and please be patient - I'll be away early next week and may not be able to respond for a couple of days.