"That was on the thirteenth day of the month of Adar; and they gained relief on the fourteenth day, making it a day of feasting and gladness. But the Jews that were in Shushan assembled on thirteenth and fourteenth, and rested on the fifteenth, making it a day of feasting and gladness." (Megilat Esther 9:17-18)
The events of Purim culminated in the Jews defeating their enemies. In general, this battle took place on the thirteenth of Adar. However, in Shushan the battle continued an additional day. In Shushan the conflict ended on the fourteenth. This salvation is celebrated through the festival of Purim. Purim is celebrated on two days. Most cities observe Purim on the fourteenth of Adar. This was the date on which most Jews rested from their conflict with their enemies. However, some cities observe Purim on the fifteenth of Adar. These cities recall, through their celebration, the events in Shushan. In Shushan, the Jews fought on the fourteenth and did not rest until the fifteenth. Which cities observe Purim on the fourteenth and which celebrate the fifteenth? Shushan was a walled city. Therefore, those cities defined as walled celebrate on the fifteenth of Adar. Accordingly, the fifteenth of Adar is referred to as Shushan Purim. Cities that are defined as open or cities without walls celebrate on the fourteenth of Adar.
How does halacha determine the status of a city as walled or open? The Talmud explains that any city that was walled at the time Yehoshua conquered the Land of Israel is regarded as walled. This criterion applies even to cities whose walls were destroyed by the time of the events commemorated by Purim. Cities that were not walled in Yehoshua's days are regarded as open cities. If the city was subsequently walled, its status remains unchanged. It is regarded as an open city. The only exception to this rule is Shushan, itself. Shushan was walled after the time of Yehoshua. Nonetheless, it is defined as a walled city. The question regarding these criteria is obvious. Why is the determination based on the city's status at the time of Yehoshua? The purpose of distinguishing between walled and open cities is to recall the different days of celebration in Shushan and other cities. It seems that a city's designation should be established by its status at the time of the Purim miracle!
Maimonides responds to this question. He explains that at the time of the miracle of Purim the land of Israel was desolate. The walls of its cities had been destroyed. If the criteria were determined by a city's condition at that time, an unacceptable outcome would result. The cities of Israel would be reminded of their fall and destitution. This would disgrace the Land of Israel. In order to preserve the honor of the Land of Israel, the determination of a city's status was based upon its condition prior to the desolation of the land. This raises two interesting questions. First, there seems to be a simpler solution to the problem of respecting the honor of the land of Israel. The creation of Shushan Purim is designed to commemorate the unique experience in Shushan. Why not limit observation of Shushan Purim to Shushan? This would avoid any slight to the land of Israel. Second, we can well appreciate the importance of honoring the land of Israel. However, we expect halacha to be governed by logical principles. Basing a city's status on its condition at the time of Yehoshua seems arbitrary and inappropriate. The fact that a city was walled centuries before the miracle of Purim is not a basis for comparing the city to Shushan!
These two questions lead to an important insight into the celebration of Purim. Purim is not merely a celebration of a past miracle and salvation. It is not solely an experience of thanking the Almighty for our salvation. Instead, it is a process of duplicating and reliving the events. For example, we fast on Ta'anit Esther and then enter the celebration of Purim in order to relive the transition from peril to salvation. The enactment of Shushan Purim serves this purpose of reenactment. It is designed to recall the various dates of salvation. Therefore, it is crucial that Shushan Purim receive prominent attention. Observing Shushan Purim in Shushan alone would be completely inadequate. This would not provide adequate recognition of the events. Instead, a set of cities was selected that assured that the two alternate days of salvation would be fully relived and recalled. We can now understand the criterion chosen by the Sages. Our Sages did not establish Shushan Purim because a city's similarity to Shushan required this alternative date of celebration. Instead, they created the celebration in order to establish a vehicle for more fully recreating the events of Purim. This necessitated selecting some group of cities to preserve and demonstrate the events of Purim. Some similarity to Shushan was necessary to communicate the message. However, a strict resemblance to Shushan was not required. Therefore, the Sages had latitude in defining the criteria for walled cities. They had the option of respecting the honor of the land of Israel. This respect did not detract from fulfilling their purpose.
Mesechet Megilah 2a. Rabbaynu Moshe ben Maimon (Rambam / Maimonides) Mishne Torah, Megilah and Chanukah 1:5.