Let's say there are villages within sight of a walled city, even if they're not close by. The village might be on a mountain, far away from the walled city, but it can see the walled city. Or they might be close by but unable to see it. Let's say they're in a valley, so they can't see the walled city, but they're very close. So long as they're within a mile's distance, they're also considered part of the din (law) of a walled city.
And Maran (our master) speaks exactly about this regarding Shushan. Even though it was not walled from the times of Yehoshua, still we celebrate on the 15th. Shulchan Aruch says the villages are within sight of a walled city—not only villages that are close to the walled city, but even cities that are close to the walled city. They're only if they're not in proximity; this is a close connection.
We said it was close, but okay. We said it was close, meaning if it's close and we can't see it—meaning it's close but it cannot be seen, like a valley—it's okay. But we can see even farther than the walled city, but we can see it even if it's not close, meaning on the mountain, for example. If you can see it even though it's far away from the walled city, it's not a walled city if you can see it even though it's far away, more than the walled city. You read it on the 15th, and there are people who can see it too, but they don't think it's far away from the walled city.
And some want to say that even if it's on the mountain, it's left to be within a walled city in order to read it on the 15th. And even our grandfathers, they rested on the 15th, like the passage says.