As we already learned (15:4), the Sages established two separate times for celebrating Purim. Most places celebrate on the fourteenth of Adar, whereas Shushan, along with cities that were surrounded by walls at the time of Yehoshua bin Nun, celebrate on the fifteenth of Adar.
It makes no difference whether the city is located in Eretz Yisrael or outside of it; any city that had a wall at the time of Yehoshua is considered a walled city, even if its wall was subsequently destroyed. Shushan, where the miracle actually took place, is the only exception; it is considered a walled city even though the city had not yet been built at the time of Yehoshua bin Nun (sa 688:1).
When the Men of the Great Assembly instituted the holiday of Purim, many large cities, especially in Eretz Yisrael, had a tradition that they had been surrounded by a wall at the time of Yehoshua bin Nun. The residents of these cities, therefore, celebrated Purim on the fifteenth of Adar. Over the centuries, however, these cities have been destroyed, and the traditions regarding their status have been lost. Some modern cities, like Lod, have the same name as cities that were known to have been surrounded by a wall at the time of Yehoshua bin Nun. The problem is that we do not know with certainty whether the modern city is located exactly where the city stood in Yehoshua’s time, or perhaps it is simply named after the ancient city but is actually situated somewhere else nearby. Some ancient cities became devoid of Jews, and we do not know whether they existed at the time of Yehoshua, because the traditions regarding their status were lost. There are other cities, such as Hebron, that we know existed at the time, though we do not know if they had a wall. There is only one city concerning which there is a clear tradition that it was surrounded by a wall at the time of Yehoshua bin Nun: our holy and glorious city of Jerusalem. Jerusalem is the only place where Purim is celebrated nowadays on the fifteenth of Adar. We will first elaborate on the laws of Jerusalem and then discuss the laws of the various uncertain places.
. Responsa Divrei Yosef (Schwartz) §2 states that Jerusalem is the only place about which we are certain that it was surrounded by a wall. Take Hebron, for example: According to Radbaz 2:681, it was not surrounded by a wall (see Teĥumin, vol. 1. pp. 122-123), whereas Ĥida writes that the Jews there had a custom to read the Megilla on the fifteenth as well (Kaf Ha-ĥayim 688:17). Regarding Lod, Megilla 4a states clearly that it is a walled city. However, it is uncertain whether modern Lod is located exactly where the ancient city of Lod stood. Therefore, we treat it as an uncertainty. This is how R. Ovadia Yosef rules in Yabi’a Omer 7:60. As for Tiberias, Megilla 5b explains that it has an uncertain status because one side of the city was not walled; it bordered on the sea. In general, even if we would discover a city that has remnants of an ancient wall from the time of Yehoshua bin Nun, it would still be unclear whether the city was built with the intention of surrounding it with a wall, as that is the only way a city can be considered walled. R. Shaul Yisraeli suggested that if the walls of an ancient city are buried beneath the ground and the houses of the modern city are built above the height of the walls, the modern city is not considered walled (cited in Teĥumin vol. 1, p. 126). See the same source for a lengthy discussion concerning the town of Beit El, which is located around a kilometer away from the excavations of the ancient city of Beit El. On p. 128, R. Sraya Deblitzky states, based on Igrot Ha-Re’iyah §423, that one should not determine halakha based on archeological evidence. See p. 130, which cites the ruling of my father and teacher, R. Zalman Melamed, that residents of Beit El should celebrate Purim only on the fourteenth of Adar.