Cheomseongdae is an astronomical observatory located in Gyeongju, South Korea, which is believed to have been built during the reign of Queen Seondeok (632-647 AD) of the Silla Kingdom. It is the oldest surviving observatory in East Asia and is considered a significant cultural and scientific heritage site in Korea.

The structure of Cheomseongdae is made of 362 granite stones stacked on top of each other, with a height of about 9.4 meters. It has 27 layers of stones in the base, with each layer decreasing in width as it ascends, and 12 rectangular stone slabs at the top, forming an open terrace. The observatory is constructed in a square shape with a width of 5.6 meters on each side.

Cheomseongdae was used to observe the movements of stars and celestial bodies, and its unique design allowed ancient astronomers to measure the length of the day, the position of the stars, and the occurrence of eclipses. Its name, which means "star gazing tower" in Korean, reflects its primary purpose.

Today, Cheomseongdae is open to visitors and is a popular tourist attraction. It has been designated as a National Treasure of South Korea and is considered one of the country's most important historical and scientific landmarks.