The Day I First Stepped onto the 'Forbidden' Hill: Science Illuminates the Intersection of Emperor Shomu's Mausoleum and the Phantom Tamonyama Castle
On February 27, 2026, a quiet historical turning point occurred in Nara City. Research organizations, including the Japanese Archaeological Association, conducted their 'first-ever' on-site investigation of the mausoleums of Emperor Shomu and his Empress, which are under the management of the Imperial Household Agency.
The Imperial Household Agency manages approximately 30,000 square meters of the area, and researchers spent about one hour and 20 minutes from around 1:00 PM surveying the outer perimeter of the two mausoleums, which are usually off-limits. You might think, 'Only a little over an hour?' upon hearing this figure. However, this is a place where researchers have been unable to even set foot for decades. That door has finally been opened.
After the investigation, Taisuke Aoyagi, a director in charge of mausoleums for the Japanese Archaeological Association, stated, 'We were able to observe things that could not be grasped from maps.' I feel that everything is condensed into this single remark.
The strange truth that 'there is a castle inside the Imperial Mausoleum'
Why does this place attract the attention of archaeologists? The key lies in the fact that this location holds a dual history: the Nara period and the Sengoku (Warring States) period.
The area overlaps with the ruins of Tamon Castle, built by the Sengoku warlord Hisahide Matsunaga, and researchers confirmed topographical undulations that could potentially be earthworks.
At the mausoleum of Emperor Shomu, terrain resembling stepped terrace-like structures can be observed, which are assumed to have served as outer baileys. There is also a theory that both mausoleums were part of the fortifications of Tamonyama Castle.
In other words, the site of this investigation is an 'Imperial mausoleum from the Nara period' and, at the same time, 'remains of a cutting-edge castle from the Sengoku period'—it was truly a dual 'sanctuary'.
Hisahide Matsunaga, the heretical architect
When discussing Tamonyama Castle, one cannot avoid its builder, Hisahide Matsunaga.
The castle was quite advanced; records indicate that a four-story turret, equivalent to a later castle keep, was built, and the construction of longhouses on the ramparts is considered the origin of the 'Tamon-yagura' (Tamon turret). Furthermore, to protect against fire arrows, roof tiles were used for the first time in a Japanese castle, and the castle walls were plastered to defend against musket attacks.
Built between 1559 and the following year, this castle was a technological leap in the history of Japanese castle architecture. Tiles for fire prevention and plastered walls for defense against gunfire—these were the crystallization of the 'latest technology' of the time. In modern terms, it was an innovation equivalent to earthquake-resistant structures or smart buildings.
Furthermore, the cultural aspect cannot be overlooked. The Jesuit missionary Luís de Almeida was invited to Tamonyama Castle in 1565 and wrote in a letter, 'I have never seen anything so beautiful in the capital, nor is there anything so beautiful in the whole world.' He particularly praised the gardens and the trees planted there, and the details remain in Luís Fróis's 'History of Japan'.
A castle that a Portuguese missionary evaluated by world standards. The fact that it existed in 16th-century Nara is, once again, astonishing.
A famous castle that vanished in just 16 years—and the double wall of 'excavation prohibition'
Tamonyama Castle was demolished just 16 years after it was built. This was because Nobunaga ordered its destruction in 1576 after Hisahide rebelled against him. This coincides with the time when Azuchi Castle was completed.
It is nothing short of historical irony that Nobunaga destroyed Tamonyama Castle, which is said to have served as a reference for his own Azuchi Castle, with his own hands.
Later, in 1948, Wakakusa Junior High School was built on the site, and the remaining earthwork ruins were lost during the construction of a new school building in 1978.
Thus, the remains of Tamonyama Castle were officially considered 'almost extinct.' However—only the area of Emperor Shomu's mausoleum, managed by the Imperial Household Agency, remained quietly untouched by anyone.
The Imperial Household Agency imposes strict restrictions on the approval of academic surveys on the grounds that 'religious services are currently being continued by the Imperial Family' and 'the preservation of tranquility and dignity is of the utmost importance.' For researchers, Imperial mausoleums have been 'sanctuaries that are close yet far'.
Topography invisible on maps—science finally begins to move
This on-site investigation means more than just a 'tour.' As the phrase 'could not be measured on maps' suggests, there is topographical information that can only be read by standing on the site itself.
Modern archaeology can capture underground ruins non-destructively by combining ground-level surveys with LiDAR (Light Detection and Ranging) and Ground Penetrating Radar (GPR). It is still fresh in our memory that LiDAR discovered the 'lost cities' of the Maya civilization and unknown city clusters sleeping around Angkor in Cambodia.
The day may be near when similar technology is utilized for Japan's imperial mausoleums as well. This 'first entry' is truly the first step toward that. If the topographical undulations observed by researchers are traces of earthworks, the next step will be verification through precise topographical surveying and remote sensing.
The significance of standing where two eras overlap
Emperor Shomu (701–756) was the emperor who ordered the construction of the Great Buddha at Todai-ji Temple during the Nara period. 800 years later, a Sengoku-period warlord built a castle on top of that mausoleum, which was even described as the 'best in Japan.' And now, 450 years after that, we have turned our scientific gaze toward that land for the first time.
History may seem complete within textbooks, but it is actually still moving. What this on-site investigation shows is the quiet possibility that 'facts not yet known are buried beneath our feet.'
What is sleeping beneath the hill of Tamonyama? The answer should be revealed little by little by the scientific investigations that are about to begin.
Reference Articles
○ First on-site investigation of Shomu Mausoleum: Nara, remains of Tamon Castle also surveyed (Kyodo News) (February 27, 2026)
○ [Nara Prefecture] The History of Tamonyama Castle: Built by Hisahide Matsunaga to rule Yamato Province, it became a precursor to early modern castles! (Sengoku History) (May 25, 2022)
○ Tamonyama Castle (Wikipedia)
○ Imperial Mausoleums (Wikipedia)