Shushan Purim (the 15th day of Adar) is the day on which Jews in Jerusalem and Shushan (in Iran) celebrate Purim. The Book of Esther explains that while the Jews in unwalled cities fought their enemies on the 13th of Adar and rested on the 14th, the Jews in the walled capital city of Shushan spent the 13th and 14th defeating their enemies, and rested on the 15th (Esther 9:20-22).
Although Mordechai and Esther decreed that only walled cities should celebrate Purim on the 15th, in rememberance of the battle in the walled city of Shushan, the Jewish Sages noted that Jerusalem, the focus of Jewish life, lay in ruins during the events of the Book of Esther. To make sure that a Persian city was not honored more than Jerusalem, they made the determination of which cities were walled by referring to ancient cities walled during the time of Joshua.
The Megillah is also read on the 15th in several other cities in Israel(such as Jaffa, Acre, Safed, Tiberias and Hebron, but only as a custom based on a doubt over whether these cities were walled during the time of Joshua).