They open the door (it slides up). It's very dark inside and once they go in, the door closes again. Once the door is closed, gravity flips upside down and they will find themselves standing on the roof. The place continues to be dark, but it feels like there's something there.

The ceiling is a maze-in-the-dark. Their own light sources will work, but can't light up other than their surroundings. They can, however see or feel that the whole maze is made of stone, much like the whole ruins are.

Their goal is to find a the exit that will lead them to climb to the next level while avoiding whatever it is lurking in the dark.

Feel free to flip a coin whether they make it to the exit or whatever there is in the dark.