The Meguilah distinguishes between Shushan and all other places: "all the Jews in the provinces, who live in unwalled cities should celebrate the 14th of Adar". While those who live in walled cities, like Shushan, celebrate Purim on the 15th of Adar.
This however, created a problem. The Holy Land lay in ruins at the time, and Jerusalem was still unwalled. Jerusalem --the eternal capital of the Jewish people-- would have had a status inferior to the other walled cities in the world. Therefore, in honor of Jerusalem and specifically to include Jerusalem, the rabbis established that "cities walled since the days of Jehousha bin Nun would be given automatically the status of walled cities". (Me'am Lo'ez)
Take the shot, a Purim inspirational from Aish
Rabbi Yosef Bitton. YMJC | 130 Steamboat Rd. | Great Neck | NY | 11024