The City is an alternate dimension in the Archive 81 universe. It is called The City because it the closest resemblance to what humans can comprehend. It is an aged, self-developed shared Dreamscape/Mindscape of our Earth, Leviathans exist in a veil between we cannot comprehend or see in raw form, and it is easier for them to manifest in a world of ideas, as concepts themselves. Weird people are more in tune with The City, the world representing the story, idea, thought, song, etc. The more weird, unnatural, or modified you are, the better your chances of survival in The City.
The City changes general appearance depending on whichever Leviathan is "winning" in the local area. Rituals are easier when doors to The City are open.
Another common feature when entering The City is that is memory loss, but specifically personal memory. Details from their own stories are missing. Rat, Suit, and the Representative of the curator forgot near everything, Caroline(Suit) a lot, Dan's are just a bit vague in spots ("unattached"? They're all still there, anyway). The exception to this is the Clerk in who is unaffected.