Cheomseongdae is the oldest astronomical observatory in East Asia, and Korean National Treasure # 31, located in the city of Gyeongju, ancient capital of the Silla Kingdom which reigned for nearly 1,000 years.
I went to South Korea on vacation in 1996. We spent about a week in the capital, Seoul. Then we travelled all along the eastern coast from north to south, stopping in Silla on our way to Pusan. It was one of the best parts of a fantastic journey.