The megillah is read on the fourteenth of Adar in cities which were not walled at the time when Joshua led the Jewish nation in conquest of Eretz Yisrael. In cities which were surrounded by walls at that time, the Megillah is read a day later, on the fifteenth of Adar.

In a city about which there is uncertainty as to whether it was walled at that time or not, its inhabitants must read the Megillah on both the 14th and 15th. This includes the ancient cities of Jaffa, Lod, Akko, Tzefat, Haifa, Beersheva, Hebron, Shechem, and Gaza, according to the Luach Eretz Yisrael of Rabbi M. Tuchichinski. Feasting and gift giving are also done on both days. The blessing on the megillah reading is said only on the fourteenth, when most of the world reads the megillah.

In Tiveriyah, too, the megillah must be read on both days. But this is not because there is any doubt that the city had walls in the time of Joshua. A passage in Yehoshua 19 describes Retkes as a walled city, and we know that Retkes is another name for Tiveriyah. What, then, is the question that arises in regard to Tiveriyah?

Tiveriyah is located on the Sea of Galilee, also called the Kinneret. Thus, it was protected from invaders by a combination of walls and the sea. If we define a “walled city” literally, as one completely surrounded by walls, then Tiveriyah does not qualify. But if we view “walled city” as one protected from invasion, then Tiveriyah’s combination of walls and sea qualifies it as such.

This is why the Sage Chizkiyahu instituted the Megillah reading on both days in Tiveriyah, a ruling cited in the Shulchan Aruch as a precedent for all cities whose status is uncertain.

(Orach Chaim 68 8:4, Mishnah Berurah 9)