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Japanese castles are classified into groups such as “Hirajiro,” “Yamajiro,” and “Hirayamajiro,” based on the terrain on which they were built. These categories reflect strategic decisions about where to construct a castle, depending on military needs and political aims.
Knowing these classifications helps you understand why each castle stands where it does and what kind of defensive strategy it was designed to support.
This article explains the five major castle types and their defining features.
Japanese castles are categorized by terrain. Based on classifications recorded by military scholars of the Edo period, the five representative types are:
In terms of elevation, the hierarchy is generally:
Hirajiro < Hirayamajiro < Yamajiro
Mizujiro and Kojo share the characteristic of using surrounding water for defense, but they are categorized separately because one relies on the sea and the other on lakes.
Hirajiro refers to castles constructed on flat, level terrain.
Because they lack the natural defensive advantages of elevation, these castles compensated with multiple layers of moats, earthen embankments, and stone walls. The gates and defensive layouts were ingeniously designed, often incorporating yagura (turrets) and fortified gate structures.
Hirajiro were not common during the Sengoku period, when military advantage was prioritized, but in the peaceful Edo period, flatland sites became preferred for castle towns, and many were constructed.
These castles feature impressive keeps, palaces, and other highlights, making them ideal for combining with castle-town sightseeing. They are easily accessible and popular for first-time visitors. There are several theories regarding the “official” set of three, including:
There is no universally fixed definition.
Yamajiro are castles built in mountainous or highland regions, relying heavily on natural topography to enhance defense.
Steep slopes and rugged terrain restrict enemy movement and make direct assaults difficult. The main enclosure (honmaru), secondary enclosures, and other compounds are arranged to fit the mountain contours, creating an organically fortified structure.
Many Yamajiro were constructed during the Sengoku period, but once the Edo period brought stability, they were gradually abandoned in favor of flatland sites that supported castle-town development.
These castles are harder to visit than flatland castles, but the panoramic views from mountaintops and the sense of historical atmosphere are exceptional. Remnants of stone walls and enclosures offer a vivid impression of their original layouts.
Hirayamajiro are castles built on hills or low mountains situated within otherwise flat plains.
While flatland castles are vulnerable to direct attack, Hirayamajiro mitigate this by positioning the main structures on elevated ground. The height provides improved visibility to detect approaching enemies. Defensive features such as moats, earthen walls, and stone walls were added for additional protection, and strategically placed gates and turrets strengthened the system.
Many Hirayamajiro were constructed during the Sengoku and early Edo periods, supporting the development of castle towns and political centers. They are considered a hybrid between Hirajiro and Yamajiro, combining advantages of both.
Some hilltop-style castles built specifically on ridges are referred to as “Ocajiro,” but they are typically treated as part of the Hirayamajiro category.
These castles balance the convenience of flatland castles with the defensive strength of mountain castles. They often feature original keeps, impressive stone walls, and broad scenic views—Himeji Castle being the most iconic example.
Mizujiro are castles constructed along the coastline, using seawater drawn into their moats as a key defensive feature. They are also known as “Umijiro” (sea castles).
Facing the sea, they used seawater moats to strengthen defenses and often included port facilities (funairi) that allowed ships to dock directly at the castle. This enabled maritime transport, logistics, and coastal military operations, making these castles strategically important points along major sea routes.
Regions where Mizujiro were constructed often flourished as hubs of commerce and culture. A particularly unique subtype is the “island-style” water castle, where an entire island forms the castle grounds—most famously seen in Noshima Castle built by the Murakami navy.
These castles offer a distinctive coastal landscape. Because their moats contain seawater, you may even spot sea bream swimming—something rarely seen at other castles. Takamatsu Castle offers sea bream feeding experiences, and Imabari Castle is beautifully illuminated at night.
Kojo refers to castles built around lakes or marshlands, utilizing natural bodies of water as defensive barriers.
By surrounding the castle with lakewater or marshes, these fortresses leveraged natural obstacles to make enemy approaches difficult. Invaders were forced to cross bridges or use boats, adding significant risk and reducing mobility during an attack.
Lake castles were designed in harmony with local geography and water systems, and many remain important cultural and historical landmarks today. While both Mizujiro and Kojo utilize water for defense, the key distinction is that Mizujiro rely on the sea, whereas Kojo rely on lakes.
These castles are known for their beautiful reflections on lake surfaces, making them highly photogenic destinations. Matsue Castle, with its original keep and charming castle-town scenery, is especially well-known.
This article has explained how Japanese castles are categorized into five types based on the terrain on which they were built, along with the defining features of each type.
| Type | Characteristics | Representative Examples |
|---|---|---|
| Hirajiro | Castles built on flat plains; defensive strength enhanced with moats and stone walls | Nagoya Castle, Hiroshima Castle, Matsumoto Castle |
| Hirayamajiro | Castles built on hills or low mountains within a plain | Himeji Castle, Tsuyama Castle, Matsuyama Castle |
| Yamajiro | Castles built in mountainous areas, using the natural topography for defense | Iwamura Castle, Takatori Castle, Bitchu Matsuyama Castle |
| Mizujiro | Castles facing the sea and drawing seawater into their moats | Takamatsu Castle, Imabari Castle, Nakatsu Castle |
| Kojo | Castles facing lakes and using lake water for defense | Matsue Castle, Zeze Castle, Takashima Castle |
Understanding these classifications helps reveal why each castle was constructed in its specific location and what defensive strategies shaped its design. When visiting castles, take note of which type they belong to—it adds a richer layer of insight to your exploration.
