So I guess that the remaining human survivors formed a nation, but what form of government exists in this nation? Who is the leader?
Seeing as there are mentions of a royal family being inside Wall Sina, it's safe to assume that the government is monarchical in nature. However, I am leaning towards a monarchical-parliamentary style, rather than just pure monarchical or parliament. Reason being, even with hints at a king being the head, the actions alone of the various commanders belie an amazing level of independence. In addition, it's been seen a couple of times that they can be summoned and tried in a public court, a trait that is definitely not purely monarchical in nature (as kings alone judge in that event). The voting system to approve a course of action mirrors a parliamentary set-up as well.