Oyya was a city located on the planet Gateway. It covered two thousand square kilometers. The Guardian of Forever was located near the city. By 2267, the city was in ruins. Spock determined that the ruins were one million years old. Archaeological excavations revealed that at one time the city was even greater in size. There were ruins of other cities on the planet, but none were as well-preserved as Oyya. (TOS episode & novelization: The City on the Edge of Forever, TAS novelization: Yesteryear)
According to another source, the city's name was unknown "...and would forever remain so." (TNG novel: Imzadi)
In one timeline, Oyya was a city made of pure crystal that resided on a high mountain. It was inhabited by six nine-foot tall, crystalline "ghosts" known as the Guardians of Forever who would greet travellers on the outskirts of their city. Within Oyya was the machines of "The Ancients" that monitored the flow of time. (TOS comic: "Harlan Ellison's The City on the Edge of Forever, Issue 1")