The set for the City of Limbo was a redress of the underground mutant city constructed for the motion picture Beneath the Planet of the Apes (1970).
In another connection with the Planet of the Apes movie series, the steps within the secret city are from the Mutant's set from Beneath the Planet of the Apes (complete with the small arena Taylor and Brent fight in). They have been painted blue and are shot from behind, but several sets show the areas on the banisters that have been overgrown by mineral deposits (becoming rock) and the area where the mutants stand while interrogating the captive Brent.
Gary Conway and Whit Bissell had worked together before in the horror movie I Was a Teenage Frankenstein (1957), which happened to be Conway's debut movie.
The prop of the clear/see thought table with various light, buttons on/in it and metallic posts that also wire with light locate in the underground city feature in this episode, previously was appeared in the eipsode "Time Lock" (Season 4 episode 8) of the TV series Voyage to the Bottom of the Sea.