No. 6 is the city that Shion and his friends live in.
It was created by the Babylon Treaty, after the last great war, most of the inhabitable areas were lost and humankind banned military power outright.
In which the six remaining states formed city-states in six regions and No. 6 and No. 5 were two of them.
Nezumi also knows about No. 6 that no one knows.