The idea of the thin stone wall between the domes, filled with secret corridors of different serpent-folk societies developed while I was drafting the map. It originally just has three domes in a big cavern, and that was kind of boring, so I worked with it until I stumbled on the idea of having not only 2 caverns (that was idea #1), but to have the wall be thick and filled with rooms (that was idea #2). I'm pretty happy with the result.
So I made a spaceship
4 hours ago