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Played 5 player last night. Ended up in a situation where my last action disc placement in Part A could only be on the city space - cant remember how and might have been wrong, but anyway.
The FAQ says
6.0 Player Actions
Q: May I place or remove a disc without taking its associated action?
A During B turns, you may remove an action disc without taking its action. You must still pay the costs for any discs to the left of your disc. During an A turn, you may only take actions that you can pay for or perform the action.
So, I was in the situation where I *could* afford one of the city tiles, but I really didnt want it. By the rules, it seems that I was forced into buying it.
So I think I may have misinterpreted the rules. This brought on other questions
1. Can I place an action on the trade line, but not trade?
2. Can I place an action on the progress line, but not take a progress tile.
The original rules state that during the "A" turns, "You may only select an action if you can perform it." The FAQ cleared up what would happen if you were unable to perform any of the available actions, allowing you to pass. Thus, to summarize the "A" turns:
(1) You can only choose an action from those actions that you are able to perform, then you must perform that action; and
(2) In the event that you cannot perform any of the available actions (due to lack of gold and/or resources), you can place your disc anywhere and pass.
Note that the only actions that you might not be able to perform would be build a city or initiate a trade (or, in rare instances, purchase a progress tile).
Also note that you are not forced to use a progress tile (such as Haven) that would then enable you to say, build a city. And conversely, you could use a progress tile before your action to put you in a situation where you would not be able to perform any action (again, such as using the Haven tile to trade your gold for a player cube).
Thus for the game situation you described, you would be forced to select and pay for a city tile (though remember that you could discard it before the Income phase). The answers to your other questions should follow from the above.