Daf Yomi - Megillah 2 - WALLED CITY FROM THE TIMES OF YEHOSHUA

Why was the determining factor for equal status with Shushan the existence of a wall during the time of Yehoshua bin Nun, and not the time of King Achashverosh? The Ran cites a Yerushalmi which explains that the purpose of this enactment was to give honor to Eretz Yisroel. At the time of Achashverosh, Eretz Yisroel was in ruins, and their cities were not surrounded by a wall. If the determining factor would have been the existence of wall during the time of Achashverosh, all the cities of Eretz Yisroel would have had the status of unwalled cities. It was therefore established that in all the cities which were surrounded by a wall during the period of the conquest of Eretz Yisroel; they would read on the fifteenth.

The Ritva states that Yehoshua was the one who led the Jewish people in the first war against Amalek. It is fitting to connect the holiday that celebrates the defeat of Haman, who was from Amalek, with Yehoshua.