The Grand Bastion was founded centuries ago by a legendary warrior named Thorfinn, after The Great Cataclysm. It's only purpose is to separate fertile lands from the Ashen wastes and it's military culture is the reason it stood so long.
Since their youngest years, children are taught to fight, survive and maintain order and after graduating school at the age of 14, they are mandated to serve in the City Watch for at least 4 years - after which they are considered adults and immediately enrolled into The Iron Legion while some of the more prominent warriors are instead chosen to join The Tunnel Fighters.
Those who resign from service in the Iron Legion or Tunnel Fighters are looked down on and not considered true citizens of the Grand Bastion, unless they do so in order to become blacksmiths or engineers.
Almost everyone in the city is a part of some Military Force, whether it's the city watch or Iron Legion - simply being rotated in and out of active duty every few weeks. When off duty the citizens have to wear their rank insignias - making those who are not a part of the Military easily distinguishable.