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)