Erthenia

Do they have mountains there?

Yes. The ones connected to those very Dungeons.

How large are the Dungeons?

Well... They go deeper than the mountains mentioned earlier. The space varies from place to place; there can be tiny passages where even one person would have difficulty passing, and there can also be spacious areas.

Are there any large cities?

No.

How do the locals treat strangers?

They don't. Few people reach them, especially those who live very deep. There, not only can you get lost, but there are also many different kinds of monsters and other unpleasant things. But if someone does manage to get there, they won't be met with aggression (unless there is a reason for it).

Do they live in families or are they mostly loners?

They may live in families until the young Assarrians grow up, but then they take care of themselves.

Is there a state with a certain structure, or is there essentially none?

There isn't.