Southeast of Tumuli-gongwon in the attractive sprawl of Wolseong-gongwon is the Far East’s oldest astrological observatory, constructed between AD 632 and 646. Its design conceals amazing sophistication: the 12 stones of its base symbolise the months of the year. From top to bottom there are 30 layers – one for each day of the month – and a total of 366 stones were used in its construction, corresponding (approximately) to the days of the year. The name literally means 'Observe the Stars Platform'.

　　Numerous other technical details relate to the tower’s position in relation to certain stars.

　　The visitor centre just outside the entrance has a digital display in English about the building’s construction.