Stratos was built in 14th century by leaders that gave their word that all inhabitants would live there. Centuries later, in the 23rd century, Stratos remained home to the Ardanan elite, and remained off-limits to all surface dwellers, known as Troglytes, with the exception of a few known as retainers. Nevertheless, Stratos was known throughout the Federation for its art and culture. The extreme luxury of the city was an example of what could be afforded on a planet with the only known source of zenite.
James T. Kirk had visited Stratos once, sometime prior to 2269, but never had an opportunity to explore the city until an emergency mission to Ardana that year to acquire a quantity of zenite was delayed allowing him time to visit. (TOS: "The Cloud Minders")
A view of the city of Stratos was depicted on several viewscreens on Deep Space 9's Promenade and Replimat accompanied by text suggesting a visit to the Cloud City or Risa. (DS9: "The Muse", "What You Leave Behind")
Stratos was designed by Matt Jefferies, who drew a "rough sketch", without ever drawing a final sketch. The city itself was created from green foam, white glue, hacksaw blades, and Xacto knives. The production designers then chopped up the foam and glued it together and put the final shapes on it, finished it off by wrapping the bottom with cotton, and hung it from the ceiling. (TOS Season 2 DVD special features)
According to John Dwyer, the metal artwork that appeared in the corridors throughout the city was metal furniture, tables, etc. that he had rented from "a guy up in Topanga Canyon," minus the glass tops. (TOS Season 2 DVD special features)
Executive Producer Manny Coto has mentioned on several occasions that if Star Trek: Enterprise had gone on to a fifth season, they would have done a prequel episode to "The Cloud Minders" which would have featured the city of Stratos. wbm