Let's say a sovereign kingdom, about the size of modern Tibet, was isolated from the rest of the world through magical means, to prevent a dark god from rising up and destroying civilisation. The plan is to seal as many of the world's authorities on destroying/containing dark gods inside as possible, put up a magical bubble, and shunt that bubble into some kind of space between realities, with the bubble automatically returning if the god dies. No-one is entirely sure if it will work, but no better option has been proposed.
This is put in place in a matter of weeks, to ensure it can't be sabotaged by agents of the god or its peers, who will be outside the bubble but too weak to reach the bubble or reverse its effect. This plan has the consent of the majority of the royal family, and some effort is made to evacuate people who really, really want out, although jingoism and patriotism is invoked to convince people to stay and fight.
This kingdom is a cultural centre of its world, at around the technology level of the late Iron Age. It's mountainous, but its staple crop grows well in the region, and there's a large valley in the middle. Magic is widespread, although effects on the level of building a bubble of reality require multiple practitioners using expendable magic sources, which are mined and need to be purified before use. From the outside, the magic barrier is obvious. From the inside, it's not - only those who travel to the edges and whose curiosity overcomes their fatigue find that the sky and mountains beyond the bubble aren't real.
The plan fails, but the god is beaten into temporary submission. While its agents are still present in parts of the kingdom, its isolation (unintentionally) robs it of a lot of its power.
How would this kingdom look just before the bubble goes up? You'd have quite a lot of people leaving, but also some coming in to be part of the Final Battle. How would the culture change as the god-killing effort goes on? Would it be likely to change character significantly - for instance, would it become more or less patriarchal?