Atlantis is a sky city that fell in a dragon attack. I have notes on this somewhere but now I can't find them, so I'll keep this brief and update it later. Atlantis fell in an unreachable part of the geography, so it still operates on the principles of a sky city, since it can only be reached by airship. It's claimed by the local territorial government but it claims to be autonomous and while the problem is being decided it's a great place for criminal activity. Captured by one government they can claim to be citizens of the other and obtain amnesty.

Atlantis is also a great source of ancient records and knowledge. Because it's so cut off from the rest of the world it hasn't advanced as quickly, and remains an artificat of bygone days. There are also rumours that that the original survivors of the fall tunneled underground, concealing many of their treasures there, but no evidence of this has ever been uncovered.

I love this concept, Jaynin!

So it is a place that welcomes visitors? Is it a dangerous place to be because of the criminal activity, or is that very much underground?

It welcomes visitors, but doesn't actually receive many. Because it's in such a hard-to-reach location (I'm thinking mountains) even airships have trouble reaching it, and it has poor docking facilities. (The entire point of sky cities is that you don't have to try and land.) Generally people only go there who really want to--archaeologists, historians, and people with something to hide.