Skifander

The Lost City of Skifander, “the Warrior Queen's Hidden Jewel”, is primarily known as the homeland of Zeetha, Daughter of Chump. It appears to be a matriarchal society with a strong warrior tradition, but most things about it are mysterious — so mysterious that hardly anyone has even heard of the place. According to Zeetha, its patron goddess is Ashtara -- "she who controls fertility." Another deity is Gwangi, who judging from the context, is possibly a warrior god or goddess. Characters with apparent first-hand knowledge of Skifander include Zeetha herself, Baron Klaus Wulfenbach, Lucrezia Mongfish, and Barry Heterodyne. Agatha and the members of Master Payne's Circus of Adventure have all heard about it second-hand. Whatever it is that Klaus knows about the place, it's enough to cause him to react strongly when he sees Zeetha for the first time. Lucrezia, by contrast, just sees Zeetha's Skifandrian roots as another thing to be exploited -- as usual. We don't know anything about Barry's views, except that the "Warrior Queen's Hidden Jewel" description got passed on from him to Agatha, so presumably his attitude toward Skifander was relatively positive.

Characters from Skifander

 * Zeetha, of course
 * Warrior Queen of Skifander in flashback (Madame Olga's telling of Zeetha's story).
 * Chump is presumably from Skifander, but we don't know anything about him except that Zeetha is his daughter.

Questions and Theories
Quite a bit more has been hinted at than actually spelled out in canon. For example, it is widely suspected that this is where Lucrezia sent Klaus, and the birthplace of his son.

If Gilgamesh and Zeetha are both from Skifander, they could even be closely related.

"The Works" contains a card showing a fairly young girl dressed in a manner consistent with the little known of Skifandrian royalty, and is described as a queen. Could she be the successor to the queen in Zeetha's story? And if so, does that make her Zeetha's inadvertent usurper?

Outside Information
Supposedly the idea of a lost matriarchal city is a common pulp or Victorian fiction trope.