In Flash #106-108 we were introduced to a complete different civilization, not living on another planet, but here on Earth.
Gorilla City is located in Africa. It occupies the same space as the jungle, but vibrates on a slightly different plane. In Flash #106, Barry Allen discovers its existence from the greatest mind on earth, a gorilla named Solovar who has figured out how to control the minds of others.
An evil gorilla named Grodd steals the secret from Solovar during a crime spree in Central City. Grodd returns to Gorilla City with the intention of becoming first its dictator, and then the leader of the world. However, Solovar contacts the Flash and the Scarlet Speedster manages to capture the super-gorilla and drive the knowledge of mind control from him.
In the following issue it is discovered that Grodd has not lost his mind control powers. We also learn of the lax nature of Gorilla City's prison security:
Gee, good thing we thought to check on Grodd's prison guard... after a week! He's escaped to a world just beneath the Earth's crust, inhabited by winged people. I can't be positive but I don't think this civilization ever appeared again in another DC story. Anyway, Grodd has used his mind control powers to take over this subterranean world and is preparing another onslaught on the rest of the world. He plans to use a "devolutionizer" ray to reduce the brainpower of the Gorilla City residents, while the strange air underground causes the air around Flash to crystalize when he tries to run too fast. Flash is briefly trapped, but manages to escape and once again find a way to subdue the Super-Gorilla.
In the final story, Barry Allen is startled to hear from his girlfriend Iris West that the Flash has a strong competitor for Central City Man of the Year. A brilliant man named Drew Dowden has made millions in the stock market and has hired workers for a factory that produces a product that nobody knows about.
But when Flash checks out Dowden, he is surprised to be attacked. It turns out that Dowden is secretly Grodd after bathing in an evolutionary ray that advances him beyond the mere humans. But Grodd reverts to his natural form and Flash defeats him yet again.
After this Grodd dramatically decreased his appearances, popping up only in Flash #115, then #127 and making his last appearance of the Silver Age in Flash #172.