A Flying City In An Anderson Future History

The Traveler before she became the first Nomad ship: 

"We had ridden centauroids who conversed with us as they went to the aerial city of their winged enemies -"

-Poul Anderson, "Gypsy" IN Anderson, The Complete Psychotechnic League, Volume 2 (Riverdale, NY, February 2018), pp. 255-270 AT p. 267.

The Nomad ship, the Peregrine:

"'The flying city on Aesgil IV, and the war between the birds and the centauroids.'"

-The Peregrine, CHAPTER XIX, p. 164.

At least two ships had visited this planet. Each of these brief accounts gives us slightly different information. Aesgil IV is either the fourth planet of a star called Aesgil or the fourth moon of a gas giant planet of that name.

There are a few flying cities elsewhere in sf. We would have liked it if Anderson had written a story about this one. However, his works were bound to include more enigmatic references than complete stories. A film or a graphic adaptation could at least present some visuals of the city and the centauroids. 

Intelligent centauroids are one possibility. Such species also appear in the Technic History.