The whole of readily habitable space is a massive tower standing in an open void. The universe’s length and width loops back around in a finite way some distance off from the sides of the tower, but the universe is of infinite height. The tower is also of infinite height, with no bottom or top, no roof and no ground floor, only infinite stacked stories. Different segments have different architectural appearances.
Beyond the tower is an empty void of geometric space. While it does contain an atmosphere, it is uncomfortably dry and sterile, and a weak gravity makes sentient beings fall towards the tower. The space itself is twisted and warped into recursive traps. There is simply no where else to go besides the tower.
The tower itself, though varied in some of its floors, is ultimately a massive rehabilitation center and prison. It thus contains mundane and supernatural criminals, many of whom are considered too dangerous for their home-plane to handle. Since the multiverse is infinite, so too is the capacity to take in convicts.
The wardens of this prison tower are a species of tentacle-face psionic humanoid. Though they seem monstrous in appearance, they are a largely peaceful and progressive society that seeks to rehabilitate even its most staunchly evil demonic inmates. They are an ancient people possessing telepathic powers to read sentient beings’ intentions and thoughts; naturally this aids, but does not solve, rehabilitation issues.
(I do not own the images in the above collage, and all copyrights belong to their respective owners.)
Go to another world …