Oyya

Oyya was a city located on the planet. 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. 

According to another source, the city's name was unknown "...and would forever remain so."