You Got Half An Hour To Leave
They tried to conceal its presence from the eyes of monsters and dark masters, eventually failing in doing so. Monsters flooded their city, killed everyone, and ruined the entire place. And that was pretty normal.
Monsters could sniff the location of any interworld portal from hundreds of kilometres away. The spatial spirit power these portals would emanate would look like candies in the eyes of any kid.
That solved the mystery of the huge ruins up there. And it seemed the monster race that controlled this region was the scary Floating Bubble monsters. They lived here to control this portal, protect it from the hands of anyone, and used its spirit power to survive and grow their numbers.
They were terrifyingly strong enough to scare most monsters away from here. They weren't just individually strong, but they were one of the rare high-grade monsters that lived in large numbers together. They were just like Scarlet Bears.