With writing you can cancel a city action of another player. Do I have to do that directly after his action or am I allowed to react later in my city action time or in the city action time of another player? In this case I would interpret the city action as a phase in which all players can do city actions like this at any time.
With writing, you can deploy a diplomat to cancel another player's city action (at least that's the mechanic in the computer version). The diplomat can be deployed at any time on one of your turns, and will operate on an opposing city action that is underway on that turn.