Alright, here's my idea in summary: A walled principality in the northern mountains, with an east-asian flair. 

The three passes into it are blocked off by massive, nigh unbreakable gates built by a previous, more advanced civilization, though reputed by the people to have been placed there by the god of earth and sky, ancestor to the princes. Withing the land is mostly untouched by the awfulness of the world. As a result, the people see their little island of civilization as the center of the world. 

When its boys reach the age of maturity, tradition holds that they be exiled from the walls, so that they might appreciate the peace within once they know the horrors without. They are only allowed to return ten years, day to day, after their exile. Despite being groomed and prepared in advance for their journeys, too many died off to starvation, exposure, and the depredations of the crazed mutants. 

As a result, the Brotherhood was formed, an association of those on their journeys that seeks to teach new exiles how to survive in the outside world. Over the years, it has morphed into a mercenary company, in the nominal employ of the Empire. Lead by a relative of the Prince, the Brotherhood has as its goals, in addition to gaining riches and experience by fighting for the highest bidder, the protection of trade caravans to the principality and the formation of ties with foreign powers, lesser as they might be.

I wouldn't be playing as the Principality itself (mostly), but rather as the Brotherhood. [@Flagg] Sounds good?