Air level of the map.

Okay, I see that you won't be convinced. I initially thought you would talk about UFOs, but you're talking about spirits.
Here's the story of air level:
Many years ago, mages created machines (air castles) that could fly in the skies. Bandits adopted the technology and created flying ships. Naturally, light ships quickly destroyed and captured air castles. The mages came up with a more advanced design - air cities and forts.