The Walled Cities of Eretz Yisrael
The Megillah is read on the 14th 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 15th 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, Tsefas, 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 14th when most of the world reads the Megillah.
In Tiberias, 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 Joshua 19 describes Rekes as a walled city, and we know that Rekes is another name for Tiberias. What then is the question that arises in regard to Tiberias?
Tiberias 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 Tiberias does not qualify. But if we view "walled city" as one protected from invasion, then Tiberias' combination of walls and sea qualifies it as such.
This is why the Sage Chezkiyahu instituted in Tiberias the Megillah reading on both days, a ruling cited in the Shulchan Aruch as a precedent for all cities whose status is uncertain.