Stratos, the capital city of Ardana.
Stratos is the capital city of planet Ardana. It is the galaxy's finest example of sustained anti-gravity levitation and magnificent anti-gravity engineering in general. Stratos is the home of Ardana's aristocratic ruling population. It is known throughout the Federation for its art and culture. (TOS: "The Cloud Minders")
A view of the city of Stratos was depicted on several viewscreens on Deep Space 9's promenade and replimat (DS9: "The Muse") accompanied by a text suggesting a visit to the Cloud City or Risa. (DS9: "What You Leave Behind")
Background
Stratos is an example of the extreme luxury afforded to a planet with the only known source of Zenite.
Executive Producer Manny Coto has mentioned on several occasions that if Star Trek: Enterprise had a season five, they would have done a prequel episode to "The Cloud Minders" which would have featured the city of Stratos.