What is the motivation behind making the event specifically require being in a province with a city? Does the presence of the city enable point defence like that to a greater extent than if the city wasn't there? Though, for the most part, I've actually used the event when I have besieged an enemy city, to be safe against relief attempts. Not sure if that's part of how the event was designed to be used. Contravallation, I suppose, but I'm not sure that was a thing during this era.