The Big Slide, but it was more of a dungeon.
Every year adventurers from all over the world climb on top of Mount Asland to enter in the Big Slide. The doors of the Big Slide only open on the march equinox for one hour, before closing in for the remaining of the year. They go in to seek riches, but also to be the one to pass the golden door on the bottommost layer. The door only open once per year, and only for one person. No one knows what lie beyond the Golden Door, because while exiting the Big Slide is easy, and indeed the losers all do to (usually) try again the next year, no-one has ever returned from beyond the Golden Door.
The Big Slide is a huge, labyrinthine slide made of sinuous brass, branching off and returning again and again in an almost eldritch manner. It goes on for hours, far below what should be naturally possible, and is full of secret passages, roughly divided into three layers:
The starting layer is where people begin when they start their journey. It is a labyrinth of brass in a claustrophobic underground, where it is easy to jump from one slide to another, and where vision is severely impeded by rocks and brass. The primary paths goes down quickly, but often secondary paths goes toward rock caverns, and even smaller and smaller corridors, until they stop entirely and unexpectedly. The Carnival of Horrors is in one such corridor, as is the legendary high path, that inexplicably travels up instead of going down, and is an useful secret passage.
The second layer is the pasture, where the slide suddenly exit rocks out of the top of a cavern thirty kilometers high and fifty long, a green pasture where the legendary Witch's house is located. The numerous branches of the slide goes down, of course, and down, and down, first in the cavern's sky, then under the pasture.
The third layer, under the pasture, look like the first, but eventually all the paths converge and stop in front of the Golden Door.