What Protected the Castle in the Sky, Takeda Castle—The Secrets of the Castle Revealed by Topographic Maps: Viewing 'Hideyoshi's Chugoku Campaign' from a Topographic and Geological Perspective [Topography & Geology of Battlefields Vol. 11-8]
The series 'Topography & Geology of Battlefields' examines 'battles' in Japanese history from a topographic and geological perspective. We are currently covering 'Hideyoshi's Chugoku Campaign'.
The Oda army, led by Hidenaga, captures Iwayu Castle and finally advances toward Takeda Castle. What kind of terrain awaits them?
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The Road to Takeda Castle
Once Iwayu Castle is captured, it is only a little further to the next target, Takeda Castle.
The straight-line distance is about 7km.
Of course, they cannot advance in a straight line and must cross the river several times.
However, if the march proceeds smoothly, it might take about 3 hours.
The Maruyama River has created a valley that is wider than those seen previously, reaching 500m to 1km in width.
Therefore, if conditions were favorable, it is likely they could have spotted Takeda Castle while marching.
After leaving Iwayu Castle, they would first cross the Maruyama River to move to the flat land on the west side (red arrow) and march along that flat land for a while.
Since this area is at least 200m wide even at its narrowest point, the risk of being ambushed was likely quite low.
This flat land continues to a point just under 2km before Takeda Castle (blue arrow), where it is believed they would have crossed the river again.
The current land use consists of urban areas and rice paddies.
One can imagine that there were likely fewer buildings and more expansive rice paddies back then.
Since it is said that Hideyoshi entered Himeji Castle around the 10th month of the lunar calendar, the season was late autumn, after the rice harvest.
They may have been able to march in a wide formation, perhaps moving through rice paddies as well as roads.
I have zoomed in on the area near the dead end of the western flat land.
While it seems possible to march along the west bank without crossing the river, the terrain is narrow and filled with obstacles like valleys and steep steps, so they likely crossed the river to use the eastern flat land.
Although this requires crossing the river twice, the visibility is good, so they probably marched while remaining vigilant as they approached Takeda Castle.
Topography around Takeda Castle
We have finally reached the foot of Takeda Castle.
Let's take a closer look at the terrain.
The mountain where Takeda Castle stands has a summit elevation of 353m, and the elevation difference from the eastern flat land is approximately 150m.
It is not a very high mountain, but the slopes are extremely steep.
The north-facing slope is relatively gentle, but for some reason, there are few paths, and one eventually has to make a large detour to the west side.
On current topographic maps, three walking routes to Takeda Castle are depicted.
Since the terrain on such a steep mountain is limited, it is likely that there is no major difference from the Sengoku period.
The Hidenaga unit is said to have consisted of 3,000 men, and they likely conducted the siege by splitting into three units to surround the castle.
While it is clear from the topographic map that the slopes are extremely steep, let's look at the cross-section to get a better sense of it.
The aspect ratio is 1:1.
The east-west slopes (AB cross-section) are indeed very steep, aren't they?
The slope angle is generally around 45 degrees—an angle that feels like a wall when you are actually standing in front of it. In fact, it is impossible for a human to stand and walk on it, and you cannot climb it without grabbing onto trees or similar supports.
Since having both hands occupied would make fighting impossible, it is highly likely that no one would have advanced anywhere other than the designated paths.
Also, while the northern slope was thought to be relatively gentle, it was only gentle up to the middle. Beyond that, it is still a 45-degree steep slope.
As expected, it seems realistic that the siege would have been conducted via the three routes mentioned above.
The Geology and Water of Takeda Castle
Geological Distribution
Takeda Castle is built on a very steep mountain. What kind of geology is it, exactly?
Takeda Castle and the surrounding mountains are all one color—pink—while the area around them is blue-gray.
The details for each are as follows.
Pink: Granite
Period: Late Cretaceous Period of the Mesozoic Era, Campanian to Maastrichtian (approx. 83.6 to 66 million years ago)
Blue-gray: River deposits
Period: Holocene Epoch of the Cenozoic Quaternary Period (approx. 10,000 years ago to present)
While the geological map above (Seamless Geological Map V2) is convenient for showing all of Japan on a single map, its accuracy is not perfect.
Therefore, there are rare cases where the details of the geological distribution do not match.
In the geological map above, part of the mountain area is shown as river deposits, but this is geologically impossible. Therefore, it is believed that the area within the black dotted line is granite.
When conditions are right, granite weathers significantly and becomes a sandy soil called Masado (decomposed granite).
In such cases, the terrain becomes gentle, especially around the mountain peaks, or it forms wide valleys where slope failure and erosion have progressed.
However, most of the mountain slopes at Takeda Castle are steep, and it is believed that they are composed of unweathered, hard granite.
The Relationship Between Geology and the Castle
Non-decomposed granite is a very hard rock; if you hit it with a hammer, it bounces back and sparks fly.
However, as shown in the figure above, the castle grounds are a flat surface of a certain size.
In other words, it is thought that stones were artificially cut out.
I repeat, granite is very hard. Therefore, I believe they drove wedges into the joints (cracks) to cut them out, but since the intervals between those joints (cracks) are relatively large (several meters apart), it is thought that this required considerable effort.
At the time the castle was built, it must have been considered an "important castle" to justify that.
Is there water?
In the NHK Taiga drama "Toyotomi Kyodai!", they won by cutting off the water supply.
So, was there actually water at the castle?
Geologically speaking, it is hard granite, so even if they had dug a well within the castle grounds (at the mountain peak), it is thought that there would be no water.
So where were they drawing it from?
Actually, I discovered something that looks like a "clear answer" on the topographic map.
There is a path extending from Takeda Castle to the south-southeast (red dotted line), isn't there?
Actually, this does not connect to the foot of the mountain.
In other words, it is a path solely for going back and forth to the castle.
When enlarged, I discovered something even more interesting.
These are likely erosion control dams (red dotted lines).
Moreover, there are four of them lined up. This indicates that it is a valley where sediment runoff (debris flow) is a concern. In other words, it shows that water is abundant there.
Blue lines indicating water flow are drawn, and I have circled the upper ends in blue.
It is likely that there are spring points at these locations or in their vicinity.
Due to the influence of water, the surrounding granite has weathered, likely leading to repeated sediment runoff.
While the current path may be a work road built during the construction of the erosion control dams, there is no doubt that water is abundant here.
Even if it were a work road, it is possible that it was an expansion of a pre-existing path.
Perhaps this was Takeda Castle's 'water source'.
In conclusion, Takeda Castle was a fortress protected by steep slopes formed of hard granite, while localized weathering into 'masa' soil and sediment runoff provide interesting clues to its 'water source'.
This concludes the 'Takeda Castle' series.
Thank you for reading.