The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at the Trope Launch Pad.
eroock
Since: Sep, 2012
Jet200
Since: Jul, 2022

Obvious example is Attack on Titan, where the world is actually much bigger than something outside of the wall and that the rest of the world is in paradise in compariasion to that small area.
Of course it doesn't have to be just "small civilization left", it's more the "there's actually a better area civilization behind the border". It was f.e. used as a joke in Total Drama Island, where behind the border of island there's a fast food place, showcasing that there is a whole town out there.