Gyeongju's  Nak-san
a.k.a.  Nang-san
Inner-Central Sacred Peak of the Old City
Still  Under  Construction

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Theoretical model of Gyeongju City at its classical peak around 800 CE -- with up to a million people
living in the entire valley -- from standing on Nak-san (Mt. Nangsan) and looking towards the northwest.  
The great tower of Hwangryong-sa dominates the landscape on the upper-right, and that's
Banwol-seong Palace on your left;  the Buk-cheon [North Stream] is at top.
Excellent evening photo by Robert Koehler of Seoul Selection
Anap-ji Pond, a royal pleasure-garden from the golden age of the 7th~8th Centuries, north of Nak-san
The Banwol-seong [Half-Moon Castle] Palace-site, NW of Nak-san, just north of Nam-san's
northern foothills, but across the Mun-cheon [Culture Shream] or Nam-cheon [South Stream] from them
.
The circular Cheomseong-dae Tower Site is seen in the upper-left, Anap-ji Pond at
upper-right, and the Gyerim Royal Forest on the right side of the Palace.
Cheomseong-dae Astronomy Tower
UNESCO:  "The Cheomseongdae Astronomical Observatory in the Wolseong Palace Belt is the
oldest existing astronomical observatory in Asia.  Constructed from 632 to 647, it was used for
observing the stars in order to forecast the weather. The stone observatory is a beautiful
combination of straight lines and curves, and was designated as National Treasure #31 in 1962.
Cheomseong-dae was built in a cylinder shape with stones. As many as 362 stones were piled up
to make 27 levels. There is a square entrance and a space to hang a ladder, about 4 meters up
from the ground.   The inside is filled with soil up to the 12th level, and the 19th, 20th, 25th,
and 26th levels all have long rocks hanging on two areas, shaped like the Chinese letter '井'.
It stands over nine meters tall and the base stone on each side measures 5.35 meters.
The four equinoxes, the two solstices and the 24 solar terms (also known as the astronomical
solar year) were determined by the observation of stars. The pavilion stone is believed to have
been used as a standard of deciding directions. The 362 stones used to build
Cheomseong-dae represented the 362 days in a lunar year.
From UNESCO:  The Wolseong Palace Belt's main monuments are a ruined palace site, Anapji Pond,
Cheomseong-dae Observatory and many others.  Royal Tomb Belt's excavations have produced rich
grave artifacts of gold, glass and fine ceramics. Most of the mounds there are domed, but some take
the form of a half-moon or a gourd. They contain double wooden coffins covered with gravel.