Earthren Mountains

See, who needs those annoying buzzing machines to keep you safe when you can just build a city into a mountain. A perfectly natural force field, none of those artificial Lightning made machines.

— A tour guide of a city build into a mountain

Maybe that doesn't work when a giant rock falling on top of you can actually kill you?

— An annoyed non-Earthren tourist

Being crushed by rocks is perfectly healthy, don't let a big force-field tell you otherwise.

Considering that even when the ground is relatively flat, Earths tend to dig head first into the ground to build their cities, it should come to no surprise that the Earthren Mountains are very much inhabited, inside and out. Many were first mined into as a source of previous gems, ore, and the elusive Enigma Crystals, but it became very clear that the various and constantly growing mountains provided a natural defence against aerial attacks and the easily collapsed tunnels gave a defence against any that tried to enter the tunnels uninvited. Given the lack of Earth Elemental's need for oxygen (at least for a few hours), this provided an extra layer against gas attacks.

While certain attacks orchestrated by a particular Firen General in the past proved that the natural defences were not infallible, they still do provide a protected place. Especially for the smaller cities, which normally would be out in the open without the resources to afford the Golden Power batteries or energy generators required to use and maintain the force fields that protect the larger and more valuable cities. As well, this allows them to maintain their industry instead of having it move away to safer cities, filling the gap as the required mining that takes place starts to run out of resources to be extracted.