Mars
Mars is the planet that the Martians fled to when they left Earth in attempt to free themselves from the oppression of the Earth Coalition. According to descriptions in the WWR Books, the atmosphere and environment of Mars remains relatively unchanged. Martian bots are designed to move with stability in the terrain whereas the Terran robots are not and are thus hindered by it. Mars' two satellites, Phobos and Deimos, both possess significant roles in the Great War due to the territories' proximity to the planet and the potential that they bear as auxiliary hubs.
Ashley Wood has clarified on 3A Forums that the Martians have built their habitat half underground:
"mars is half and half, much in underground, with shielded structures on top. there isnt any full on breathing on mars, the atmosphere is still too thin, a little too much radiation for the exposed skin." [1]