Backstory isn't irrelevant. It makes for an important aspect of the game. It's one point of triangulation for developing plots, with the other being the players' characters.
For example, if I were to set up a campaign in the Iron Kingdoms' city of Five Fingers, the backstory involving the Greylords' Covenant is likely to come into play. Why? Because that backstory influences their goals, their methods, and their rivalries. And it influences their role in any plots. Add in a player character who has connections to either the Covenant or their rivals, and boom. You're now capable of creating plots that involve both.
In your case, this city in a bubble sounds like it would be contain a lot of wizards, or at the very least a lot of arcane artifacts and baubles. Very high magic. Depending on what happens to the city between the original reason and the present, you could go a lot of ways with it. Including where this bubble is located, what sort of intrigues motivate its inhabitants and the people who outside of it yet interested in it, and so forth.
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For example, if I were to set up a campaign in the Iron Kingdoms' city of Five Fingers, the backstory involving the Greylords' Covenant is likely to come into play. Why? Because that backstory influences their goals, their methods, and their rivalries. And it influences their role in any plots. Add in a player character who has connections to either the Covenant or their rivals, and boom. You're now capable of creating plots that involve both.
In your case, this city in a bubble sounds like it would be contain a lot of wizards, or at the very least a lot of arcane artifacts and baubles. Very high magic. Depending on what happens to the city between the original reason and the present, you could go a lot of ways with it. Including where this bubble is located, what sort of intrigues motivate its inhabitants and the people who outside of it yet interested in it, and so forth.