Enigmundia: A Hollow World as a Labyrinthine Prison

Mystara has a hollow world, and according to canon, it was created and is sustained as some sort of dynamic stasis world for civilizations and creatures that were going to become extinct or be lost.

For Enigmundia, especially the Kingdom of the Wheel aspect of the setting, this can be conflated into a prison / menagerie of multiple gaming worlds within Mystara that are folded and interlocked into themselves like a mad combination of origami and mobius strips and gordian knots. I can have in that world -- segregated by Immortal class spells, mini-settings like Thunder Rift and the Lendore Isles, very specific culturally inspired settings like Harn and Roma Imperious, and larger distinct settings like Dark Sun and the Majestic Wilderlands and Ravenloft.

And I'd probably stick Vornheim in there as the gateway city or the city that can be reached from almost any of these prison setting locations. That's also  probably where you can buy a map (that looks something like the one from Time Bandits) to travel to different places in the Hollow World.