Mountain Temple Castles (2)
──The Military Conversion of Religious Spaces and the Original Image of Sengoku Castle Construction Technology
──Re-examining the "Common Sense" of Mountain Castles
Mountain Temples as the Prototype of Mountain Castles
For many years, research on mountain castles (yamajiro) in medieval Japan has relied on the conventional view that they were "built by samurai from scratch, clearing undeveloped mountainous areas based on the military requirements of the Sengoku period." However, recent progress in archaeological excavations and research into nawatari (castle design and layout) is forcing a fundamental reconsideration of this typical samurai-centric historical view. In the actual historical process, it was not the samurai class, but rather mountain temples—which had been expanding their own influence from ancient times through the medieval period—that possessed the know-how to clear rugged forests and create flat surfaces through large-scale civil engineering projects.
An analysis of the Tendai-sect mountain temple complexes represented by medieval Omi Province (modern-day Shiga Prefecture) reveals that they overwhelmed early local samurai in terms of the scale and technical capability of their civil engineering work. The technology for developing sloped land in mountainous areas, vast financial resources, and the ability to mobilize thousands of people were all accumulated on the temple side. When late-coming samurai lords built defensive strongholds in mountainous regions, it is not hard to imagine that it was far more rational to incorporate and modify the temple grounds—which were already completed as highly advanced infrastructure—and "militarily convert" them into their own castles, rather than clearing undeveloped mountains from scratch themselves.
In this way, a structural continuity emerges: the prototype of the mountain castle was not an original creation of the samurai, but was established through the imitation of the preceding "temple ground system" of mountain temples, or through the spatial takeover of those sites.
Military Conversion of Approach Structures—From "Spaces for Gathering People" to "Systems for Consolidating Military Forces"
The spatial feature that made mountain temples the prototype for castles lies in the layout of their "approach paths" (sando). The temple ground structure typically seen in medieval mountain temples in Omi is built upon a framework of "linear approach paths" laid out from the mountain foot along valley lines, with flat surfaces (equivalent to baileys) called "sub-temples" (in-temples that make up the main temple complex) arranged in a continuous, stepped fashion on both sides, ultimately connecting and converging at the "main hall flat surface" at the highest point.
In conventional castle research, the existence of such "main roads leading straight to the center" or "groups of flat surfaces with nothing to block them on either side" has been regarded as a fatal weakness (disadvantage) in terms of defense. Nevertheless, when samurai converted these mountain temples into castles, the structure of the approach paths and the layout of the sub-temple groups were used almost exactly as they were, with little modification. This is because a different dimension of "practicality," distinct from military defense, was prioritized.
Mountain temples were not originally designed as "defensive facilities" to keep enemies out, but as "spaces to efficiently gather believers and practitioners from a wide area and lead them to the main hall." And in the Sengoku period, this centripetal customer-gathering system was seamlessly shifted into a "military consolidation and deployment system" for rapidly mobilizing military forces, instantly transmitting orders to each bailey (sub-temple), and efficiently transporting supplies.
Functional Transformation of Spatial Structures in Temples and Castles
【Original Functions in Mountain Temples】
Linear approach path: A flow line that leads visitors solemnly from the mountain foot to the main hall without them getting lost.
Sub-temple flat surface: A living space where each sub-temple belonging to the temple organization conducts individual economic and religious activities.
Main hall flat surface: The final destination where all visitors gather, and where the principal image, the object of faith, is enshrined.
【Military Conversion Functions After Becoming a Castle】
Linear approach path (Main gate road): A military road that pulls mobilized troops, provisions, and weapons up to the center all at once.
Sub-temple flat surface (Stepped bailey group): Tiered positions where clans, vassal groups, and foot soldiers can be immediately deployed and stationed.
Main hall flat surface (Main bailey): A command post where the general sets up camp and centrally manages the supplies and troops coming up from the mountain foot.
At Kannonji Castle in Omi, it has been proven that the structure has multiple linear approach paths (such as the Akasaka Road) that converge just before the main hall flat surface, and that the centripetal route design technology cultivated by mountain temples was inherited directly as the core part of the nawatari (grand plan) of the massive Sengoku castle.
A Shift in Perspective: From Warrior Monks to Technical Groups
When discussing the relationship between medieval temples/shrines and "military power," much historical research and fiction has tended to focus solely on direct instruments of violence such as "warrior monks (sohei)" or "armed sectarians." The two major monastic groups seen in the regulations of Taizan-ji—the "gakuryo (scholars)" and "doshu (lay practitioners)"—or the semi-monastic, semi-secular groups known as "sato-so" or "sato-hoshi" who frequented the area, were certainly powerful military forces for the temple. However, the more essential historical turning point during the Sengoku period was not the supply of such troops, but rather the "supply of castle-building technology itself," which the temples had monopolized, and the subsequent "absorption of these technical groups into samurai power."
Throughout the medieval period, the creation of large-scale forests, the construction of massive wooden temple halls, and even the techniques for building stone walls to prevent landslides and manufacturing highly fire-resistant roof tiles were all protected and passed down through the powerful economic strength and privileges of the temples. The craftsmen responsible for these—stonemasons, tile makers, and master carpenters—were only able to demonstrate their skills under the protection of the temple organization.
However, as Sengoku daimyo rose to power and began to erode the old temple and shrine estates, the control over these technicians also shifted to the samurai. The daimyo began to take into their own hands the "appointment of master carpenter positions (daikushiki-bunin: the authority to appoint and certify the general supervisor of construction)," which had previously been monopolized by temples and shrines.
Transformation of Craftsman Organizations and the Shift to Daimyo Power
[Traditional Control (Temple-led)]
Shiki-no-daiku (Official Carpenters): Traditional carpenter families and lineages officially recognized by temples and shrines. They inherited their privileged status.
Daikushiki (Master Carpenter Position): A qualification certificate issued against the backdrop of temple and shrine authority. It became a vested interest and was even subject to buying and selling.
[Sengoku Period Control (Daimyo-led)]
Toki-no-daiku (Temporary/New Carpenters): Technicians who, despite being newcomers, possessed real ability and were directly employed by daimyo to oversee construction sites.
Bureaucratization of the Master Carpenter Position: Daimyo seized control of qualification issuance, dismantled hereditary privileges, and organized technicians based on merit.
The most prominent concrete example of this phenomenon of the transfer of technology and craftsmen is the "Anoshu," a group of stonemasons based in Sakamoto, the temple town of Enryaku-ji on Mount Hiei. Originally, they were a group of technicians responsible for building stone walls for Mount Hiei and the surrounding satobo (retirement temples), and they preserved the traditional technique of "nozurazumi," which skillfully combines natural, unprocessed stones. This technique boasted extremely high drainage and earthquake resistance, and was intended to protect the grounds of mountain temples from landslides.
When Oda Nobunaga burned down Mount Hiei in 1571, the defensive power of the sturdy "Anoshu-style" stone walls surrounding Enryaku-ji caught Nobunaga's eye. The fact that they were later recruited as magistrates for stone wall construction during the building of Azuchi Castle became the greatest turning point in the history of castle architecture. The "all-stone-walled castle," synonymous with Oda-Toyotomi style castles, was not a technology developed by samurai, but was born as a result of Sengoku daimyo forcibly seizing and requisitioning the infrastructure maintenance technology of mountain temples along with their entire systems.
Not "Defense" but "Survival Strategy"—The Reality of Taizan-ji's Multi-layered Diplomacy and Fortification
The phenomenon of mountain temples becoming castles, or functioning as mountain fortresses, is often discussed in a combative context, such as "religious forces arming and fortifying themselves to resist secular rule." However, tracing the historical trajectory of "Taizan-ji," a major Tendai sect temple in Harima Province (modern-day southern Hyogo Prefecture), reveals a completely different logic of survival: a flexible diplomatic strategy of "becoming a castle (encampment) not to fight, but to survive."
The period when Taizan-ji most spectacularly exercised military power was in 1333 (Genko 3) during the Northern and Southern Courts period, when it responded to the imperial decree of Prince Moriyoshi, the Tendai Zasu (head of the Tendai sect), and joined forces with the Akamatsu clan of the Southern Court to defeat the Kamakura Shogunate's army. However, after this peak, the temple's influence entered a period of rapid decline. In the late Muromachi period, estate income plummeted, and the temple had to constantly seek connections with local powers that would protect its temple lands.
Under these circumstances, the actions taken by Taizan-ji were thoroughly focused on "parasitism and reconciliation with the powers of the time."
Under the protection of the Akamatsu clan: From the Kakitsu Incident to the Onin War, by receiving the external protection of the Akamatsu clan, the shugo (military governor) of Harima, the temple positioned its own interests within the shugo-kokusei (provincial military governor system).
Coordinated action with the Akashi clan: After the fall of the Akamatsu clan, they moved quickly to align with the Akashi clan, a powerful local samurai group in Harima, thereby securing safety and security within the local community.
Providing a base (encampment) for Miyoshi Nagayoshi: In the 23rd year of the Tenbun era (1554), when Miyoshi Nagayoshi invaded Harima to defeat the Akashi clan, Taisan-ji Temple offered its own temple grounds as an "encampment" to Nagayoshi without resistance. At this time, the branch temple "Zukosan Taikoku-ji," which was said to have directly resisted the invading army, followed the harsh fate of being burned to the ground, but the main temple, Taisan-ji, avoided destruction by becoming Nagayoshi's encampment.
Total submission to Hashiba Hideyoshi: During Hideyoshi's invasion of Harima starting in the 5th year of the Tensho era (1577), Taisan-ji chose to follow the overwhelming military power of the Oda forces immediately. As a result, they obtained a "kinsei" (a document prohibiting violence and looting within the temple grounds) from Sengoku Hisahide, a close aide to Hideyoshi, and succeeded in avoiding ruin by fire.
In this way, for Taisan-ji, "becoming a castle" or "becoming an encampment" was a sophisticated diplomatic "survival strategy" to ensure their own existence and privileges by willingly offering their powerful religious and spatial infrastructure to secular victors as "military assets."
“Great Temples That Became Castles” and “Great Temples That Did Not”
Regarding the fortification of mountain temples, not all temples followed the same fate during the Sengoku period. Some temples transformed into full-scale mountain castles, some maintained their purity as religious spaces, and others were reduced to ashes as a result of militarization. What were the factors that determined the fate of these "temples that became castles" and "temples that did not"?
The table below compares cases of major great temples in Omi and Harima, looking at their geopolitical location, relationships with those in power, and the path to fortification.
Nyosan Myoyo-ji Temple - (Harima, Kita-ku, Kobe) - A key transportation hub connecting Miki and Arima, located at an altitude of 515m. - Closely cooperated with the Bessho clan, lords of Miki Castle. - Fortified as an advance fortress (Nyosan Castle) to secretly supply provisions to the Bessho clan. - In the 8th year of the Tensho era, it incurred the wrath of Hideyoshi's army and was destroyed by fire, suffering the tragedy of even the children being massacred.
Taisan-ji Temple (Taisan-ji Castle) - (Harima, Nishi-ku, Kobe) - A hilly area overlooking the Akashi River basin, near the border of Harima and Settsu. - Multi-layered alliances with the Akamatsu, Akashi, Miyoshi, and Hashiba Hideyoshi clans. - Did not resist, but voluntarily provided an encampment to the power of the day (Miyoshi and Hideyoshi). - Escaped the ravages of war and had its temple lands confirmed. Succeeded in maintaining its religious lineage to the present day, despite declining.
Yako-ji Temple - (Omi, Maibara City) - Located at the southern foot of the sacred Mt. Ibuki, a key point for border defense connecting northern Omi and Mino. - The Shugo Kyogoku clan intervened directly. - The Kyogoku clan directly requisitioned the entire temple area on the mountain and constructed a group of massive ridge-shaped dry moats behind it. - The castle and temple coexisted until the Tenbun era, but the function was completely transferred to a military base at the end of the Sengoku period.
Shimizu-ji Temple (Shimizu-yama Castle) - (Omi, Takashima City) - On the west coast of Lake Biwa, in the southeastern part of the Aibano plateau, which commands the West Omi Road. - The head family of the local powerful clan, the Sasaki Etchu clan (Takashima Shichito). - Converted the grounds of multiple temples at the foot of the mountain, such as Shimizu-ji and Daibo-ji, into "samurai residential areas of the castle (West Residence)." - In the 4th year of the Genki era, it surrendered and opened its gates as Kido Castle and Tanaka Castle (Shimizu-yama Castle) due to Oda Nobunaga's conquest of Takashima District.
From this comparative analysis, the boundary line that determined whether a mountain temple transformed into a "castle" can be summarized in the following three points.
Geopolitical and strategic location: For daimyo and local samurai, it was extremely important whether the temple was located at a junction of roads, a border choke point (checkpoint), or a position that controlled the supply line for provisions during a siege.
Diplomatic stance and solidification of alliances: Temples that became a community of fate with specific local samurai (such as the Bessho clan), abandoned neutrality, and committed fully to the war (like Myoyo-ji) invited all-out burning by opposing forces (such as Hideyoshi). On the other hand, temples like Taisan-ji that adopted a flexible alliance strategy of lending their encampment to the "winning horse" each time were able to survive.
Topography of the temple grounds: Terrain that had a ridge behind it to be cut off (room to dig a large moat) and large flat areas (groups of sub-temples) already created on the hillside made it extremely easy for samurai to requisition and modify the site into a castle layout.
In contrast, "great temples that did not become castles" throughout the Sengoku period were limited to rare cases that were intentionally located in deep mountains and valleys away from major traffic routes, or that successfully maintained "neutrality" by thoroughly asserting a religious "barrier (demilitarized zone/sanctuary)" against secular power, refusing to provide encampments or supply provisions to any samurai family.
Challenging the common sense that “castles are on mountain tops”—The theory of mid-slope centers and main hall as the main enclosure
In conventional Sengoku mountain castle research, and by extension the image of "castles" held by the general public, the mountain-top-centric view that "the center of the castle (main enclosure/honmaru) is at the top of the mountain, and from there, ridges are cut in four directions to expand the baileys" has been taken as a self-evident truth. However, when observing castles built based on the grounds of mountain temples, this view of mountain castles is completely overturned.
The greatest feature of castles originating from mountain temples is that the center of their actual function is not at the mountain top, but on the "flat surface of the main hall on the mid-slope" or the "valley-like terrain on the mid-slope."
Kozutsumi Shiroyama Castle (Omi): The vast flat surface on the mid-slope (identified as the former main hall area) reached by climbing the straight approach from the foot of the mountain serves as the main enclosure (living and administrative space), and the highest point, the mountain top, is reduced to a very limited function as a "turret base (lookout)."
Kannonji Castle (Omi): The summit is merely a relatively small, flat area, and the actual center is the large, terraced flat area (the former site of the Kannonji main hall) that spreads out directly below it to the southeast. Even after it became the residence of the Rokkaku clan, the main route from the outside was laid out to pass through this central flat area, meaning the connectivity of the temple-style passageways outweighed the defensive isolation typical of samurai fortifications.
Yatakaji Temple (Omi): Even after being converted into a castle, the old main hall area on the hillside ridge remained the spatial core, with the flat areas of the sub-temples spreading out below it like stars. The defensive line was not set at the highest point of the summit, but along the outer edge of the temple grounds (the ridge line) so as to encompass all these hillside facilities.
Comparison of typical castle structures
[General Sengoku period mountain castle (summit-centered model)]
Main enclosure (Honmaru): Located at the highest point of the summit. Aimed at maximizing visibility and defensive power.
Structural development: Enclosures are expanded downwards from the summit in a radial or sequential pattern.
Defensive design: The main castle approach is bent at a 'koguchi' (gate) to allow for shooting and blocking from above.
[Mountain castle derived from a mountain temple (hillside/main hall-centered model)]
Main enclosure (equivalent to Honmaru): Located on the flat area of the main hall on the hillside (a centripetal point where supplies and personnel gather).
Structural development: The summit remains only as an auxiliary watchtower base or a symbolic space as a sacred site (sutra mound/inner sanctuary).
Defensive design: Inherits the straight temple approach as is. Emphasizes the continuity and practicality of individual flat areas (sub-temples).
This structure of a 'hillside-centered, main hall as the main enclosure' also casts a significant stone into the chronological view of the history of Japanese mountain castle development.
General academic theory holds that mountain castles in the first half of the Sengoku period were merely simple forts on summits, and that they evolved into complex 'multi-enclosure mountain castles' by the end of the Sengoku period through large-scale leveling of slopes and ridges. However, in castles based on mountain temples (such as Mizuki Okayama Castle and Shimizuyama Castle), large-scale area divisions and hierarchical groups of flat areas (temple sites) from the summit to the hillside and foothills were already completed by the first half of the 16th century (the first half of the Sengoku period). This is nothing less than the result of temple technology, which was ahead in mountain development, bringing an 'early developmental stage' to samurai castles.
Only by abandoning the perspective of viewing castle history solely from the summit and re-examining it from the religious space on the hillside can we grasp the true formation process of Sengoku mountain castles and the dynamic transition of the militarization of medieval urban spaces.
Summary: A landscape of layered prayer and defense
The historical trend of the 'fortification' of mountain temples from the medieval to the Sengoku period, and their 'conversion into mountain castles' by the samurai, is not just a scene from a barbaric era where religion and military force colluded. It was a process of inheriting layered technology and infrastructure, where the space of 'mountains' in Japan dynamically transformed from sacred sanctuaries of prayer into bases of military and political control.
The samurai did not carve out the mountains and build castles solely by their own power. The foundation stones of the massive fortresses they acquired, the orderly rows of terraced enclosures, and the magnificent main approaches leading to the summits were all 'temple infrastructure' that monks, believers, and nameless craftsmen had built over centuries for prayer and ascetic practice. And the presence of craftsmen, represented by the Anoshu who operated behind the scenes, functioned as a decisive 'bridge' that passed the genes of castle-building technology to the next generation as the ruling power shifted from temples and shrines to the samurai class.
It was not a defensive facility built for fighting, but a survival strategy of providing flexible encampments chosen to survive. A rational living space design that placed the actual site of life and command in the main hall on the hillside while still revering the summit. These facts pleasantly shake the fixed idea of 'samurai castles' that we hold today.
Today, when we stand at the ruins of a mountain castle and look up at the deeply carved moats and the dry-stone walls, what we see is not just the shadows of samurai who competed for hegemony in the Sengoku period. The layered breathing of medieval mountain religious practitioners, who once lived on those mountainsides, let the sound of sutra chanting resonate, and built sanctuaries while listening to the voices of the stones, can certainly be heard from beneath the soil.
This content was created based on Gemini through repeated dialogue with the contributor.
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