Bavli, M'gila 5 amud 2, cites a doubt as to when the m'gila is read in T'verya: on the fourteenth of Adar, as in most places, or on the fifteenth, as in a city walled since the time of Y'hoshua. The doubt arises because T'verya is walled except for its sea-facing side: is that side considered walled, since it's protected, or not, since it is open to the outside? In fact, as codified in Mishna B'rura 688:9, T'verya reads the m'gila both days, for this reason.
So a sea-facing side is a doubtful wall.
Consider Manhattan. Assume for the sake of argument that it's been an island since the time of Y'hoshua. Then it's been doubtfully walled since the time of Y'hoshua and so should read both days. Yet I've never heard of anyone reading two days in Manhattan — or on any other island.
- Why not?
- Is it perhaps because the city must have been inhabited since the time of Y'hoshua? If so, is that rule stated anywhere? I've never heard of it. And then what about the island-city of Kythnos, which has been inhabited that long? If Jews settle there, would they read both days?
- Do any halachic opinions maintain that islands (that have been islands since the time of Y'hoshua) should read both days?