Unlike all other Yamim Tovim, which are observed on the same day by the entire Jewish People, why did the Sages institute Purim on different days for different cities?

What additional miracle is commemorated by observing Purim on the 15th of Adar?

Why is the status of a walled city determined by the times of Yehoshua Bin Nun? Optimally, should a city in Eretz Yisrael be walled, or unwalled?

Are the rules that define a walled city for Purim, identical to those defining a walled city for redemption of houses (in Maseches Erchin)? Must the city have been walled before its being settled?

What are the criteria for when a walled city's neighboring towns observe Purim along with it on the 15th of Adar? When do the new neighborhoods of Yerushalayim, outside of its walls, observe Purim? What about neighboring cities from which one can see parts of Yerushalayim?

What cities observe Purim on the 15th of Adar? What cities have observed both days out of doubt?

What sort of doubt about a city's status serves as sufficient grounds to observe both the 14th and 15th as Purim? When does a city in doubt recite Berachos over the Megilah, and on what day do they say Al ha'Nisim?

How do we derive that a visitor observes Purim along with the city he is visiting (provided he plans to stay over)?

In order to observe Purim with the city he will be visiting, at what hour must he be located there? Is it sufficient to intend to be there at that hour, and at what time must he have that intention?

Is it possible to become obligated in both days of Purim - in an unwalled city on the 14th, and again in a walled city on the 15th? May he recite Berachos on the 15th as well?