What I mean is a city, nation, region, or other demarcated area that maintains, or attempts to maintain, a self-imposed isolation from the rest of the world.
It doesn’t have to resemble the real world DPRK at all internally, only an ongoing policy of self-reliance or exclusion.
My D&D world example would be Jaanos, which is a very small city-state that consists of a walled library. Their society is egoist-anarchist, meaning they have no central government of any sort (nor written laws) and all of the “citizens” cooperate or compete with each other in the pursuit of gathering as much hidden and obscure knowledge as possible. They’re extremely careful about who they allow inside and even more careful about who they allow to join as citizens. Their library has almost everything they need inside of it to live: wells for water, gardens and very small livestock cultivation for food, and many years of stored goods. They participate in some amount of trade for things they can’t make themselves by offering espionage services, or simply access to the library itself, but can go for very long periods of time without needing to do so.