The study of royal tombs of Silla has been limited to the superficial level due to the difficulties excavating and investigating and the limitation of resources. Therefore, most studies have discussed the Twelve Zodiac Animal Deities, the structure of stone circles, and the surrounding stone artifacts. However, even those studies were not complete due to the realistic limitations because the royal tombs cannot be explored.
Fortunately, recently excavations have identified a number of pieces from the stone circles used for royal tombs and it has been found how the stone circles were processed and jointed for the royal tombs. Therefore, this study examined the structure of stone circles around the royal tombs.
Starting with the Royal Tomb of King Sinmun, stones circles were put around the mounds. The stone circles composed of Jidaeseok(foundation stone), Myeonseok(wall stone), Taengseok(stone pillar), and Gapseok(capstone) became the basic structure of royal tombs since the Royal Tomb of King Seongdeok. Each stone circle looked slightly different from tomb to tomb, but it shifted from simple shapes to additional processing. Also, the Twelve Zodiac Animal Deities were engraved on Taengseok and its shape varied from tomb to tomb. Each royal tomb's Taengseok with the Twelve Zodiac Animal Deities included the number of stone circles in the multiple of 12 to place around the royal tombs and arrange differently according to the scale and size of stone materials. Taengseok of the Twelve Zodiac Animal Deities is thinner and longer for Pyeongbok(civilian clothes) than for Mubok(military uniform) and that of Mubok was produced in the same proportion.
In case of the royal tomb site to the east of Hwangboksa Temple Site, the stone circle and Taengseok of the Twelve Zodiac Animal Deities excavated by the recent survey were from at least two different royal tombs and produced around the time of Royal Tomb of King Gyeongdeok considering the connection of Gapseok.
As a result of analyzing the structure of stone circle of each royal tomb, the styles were classified by scale, number of stone circles, arrangement of Taengseok of Twelve Zodiac Animal Deities, and shapes of Gapseok and Jidaeseok to assume that the structure of stone circle developed in the following order: Royal Tomb of King Sinmun - Royal Tomb of King Seongdeok - stone circle from Royal Tomb Site to the East of Hwangboksa Temple Site - Royal Tomb of King Gyeongdeok - Twelve Zodiac Animal Deities from Royal Tomb Site to the East of Hwangboksa Temple Site - Royal Tomb of King of Wonseong - Royal Tomb of King Heondeok - Royal Tomb of King Heungdeok - Tomb of Kim, Yoo Shin - Royal Tomb of Queen Jindeok - Square Tomb of Gujeong-dong.
Based on the findings, the shapes of each stone circle, and the literature records, currently identified royal tombs were reexamined. Starting with the Royal Tomb of King Sinmun [Royal Tomb of King Hyoso] that began installing stone circles around the mounds, the owner of the royal tombs was assumed in the order of development of stone circle ― Royal Tomb of King Seongdeok - stone circle from Royal Tomb Site to the East of Hwangboksa Temple Site [King Hyoseong or King Seondeok] - Royal Tomb of King Gyeongdeok - Twelve Zodiac Animal Deities from Royal Tomb Site to the East of Hwangboksa Temple Site [King Hyegong or King Seondeok] - Royal Tomb of King of Wonseong - Royal Tomb of King Heondeok - Royal Tomb of King Heungdeok - Tomb of Kim, Yoo Shin[King Huigang?] - Royal Tomb of Queen Jindeok [King Sinmu] - Square Tomb of Gujeong-dong [King Gyeongmun].
The study was focused on the shapes of individual stone materials that compose the stone circle of royal tombs, so it discussed the locations of royal tombs, stone materials of tomb areas, and images of Twelve Zodiac Animal Deities only a little bit. The limitations of this study should be supplemented with more resources and further studies.