Five College Center for East Asian Studies Study Tours Website
"Ch'eomseongdae (designated National Treasure #31) means "Star Gazing Tower." Built during the reign of Queen Seonduk in 634, it is the oldest existing observatory in the Far East. It has 27 levels of stones in a round shape (Queen Seonduk was the 27th ruler of the Shilla Dynasty) with four sets of parallel bars to make a square-shaped structure on its top. The ends of the parallel bars jut out several inches from the surface and might have been a support for a staircase used to reach the top.
The 12 rectangular base stones are positioned in a square, three on each side, representing the four seasons and twelve months of each year. The twelve tiers of stones to the window entrance and twelve tiers above the window opening also represent the 12 months of the year (or the 12 symbols of the zodiac). For over two thousand years, astronomers studied the movements of stars and planets and charted them. They predicted solar and lunar eclipses, as well as the courses of comets. Court astrologers reported and interpreted their findings to the King who would then act according to their predictions.
The stars dictated all aspects of policy making: agricultural developments, celebrations, wars, and other events and festivals. Astrology also shaped people's views of life and the universe. Koreans still tend to believe that the exact time of birth for each person is closely governed by the heavens." (Life in Korea Website)
Wikipedia - Cheomseongdae