(Date Unknown)
Nemo built the city. No account survives of when or how. The oldest Carceri structures predate every other species' arrival, and whether Nemo created the Carceri or the Carceri were already here when Nemo began to build is as much a question of theology as of history and the entity that could answer it has no interest in doing so.
What is known is that Nemo established two things simultaneously: the city and the rule. Nod would be neutral ground. External conflicts between Aeons would not be prosecuted within its borders. The enforcement mechanism for this rule was tested exactly twice and none of the Aeons involved survived into recorded history. In the duration since every other Aeon has adhered to the rule.
Every institution that has formed since, from the Council to the Guilds exists because Nemo permits it. Permission has never been formally granted, it has simply never been revoked.