What is Hub World?
1.
A level in a video game which links to all the other levels, and serves as a main area that the player will use to progress through the game. It will usually be blocked off to later levels until specific conditions are met (Such as "Collect X amount of Y."), but in some cases, the portal to the level is blocked off, but clearly visible (Such as Glover). The hub world can have more than portals to different worlds, but that is the base use of the level.
Gruntilda's Lair is a Hub World.
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