1).- “ABOUT WHETHER THEIR WALLS WERE CALLED RETAINING WALLS (Short answer: No):”
Well, technically they can’t be called retaining walls (Yōheki), because that’s done for natural land elevations. The Japanese literally “built” these Mounts, for defensive purposes, and then contained them with walls. In fact, I think the most accurate translation for Japanese defensive walls is “Earthworks” or Dorui in Japanese.
2.- “JAPANESE EARTWORKS WALLS OR DORUI.-”
The Dorui, were defensive constructions, technically they were artificial elevations of land, with a trapezoidal edge of carved stones, and an interior of excavated earth. Unlike the European, African, Mesopotamian, Asian versions (practically almost the entire world); the Japanese did not develop mortar and lime, which is earth liquefied with lime to make it conform to the shape of the mold.
However, the fact that they did not use mortar and lime does not mean that they were not considered walls, just that “*they were weaker” and had to use a Trapezoidal shaped stone support base to contain the interior.
As a curious note: contrary to what some blogs say, Japanese castles or fortresses didn’t make their walls trapezoidal to be climbed, they simply “didn’t know the technology to make them straight as wll as wide”
On the other hand, there is also the term Ishigaki to refer to stone walls, but without filling, as if we were talking about a brick wall. Because they lacked the technology for straight lines to be longer than 2 meters, eartwork were prefered. Most stone walls in the Sengoku Era were part Earthworks or Dorui covered with precisely carved stones, which could also be referred to as Ishigaki, and they are the best known if we talk about the Sengoku Era.
3).- “ABOUT WHETHER THE JAPANESE WALLED THEIR CITIES:”
Like the French and the English, not all the towns and farms were protected by walls, only their most important cities, but even so I don’t think we should take away the possibility of building stone walls (To the English and French hehe). On the other hand, in the game there is no building called “castle”, but the Keep, which must have buildable walls to be a castle, as shown in the English campaign. The Japanese had the “Dorui” as walls for their castles, and of the village castles, where samurai lived, there were barracks and other buildings apart from the keep. The most notable example is Edo Castle, so in theory I don’t see any problem with stone walls.
Regarding the defense of minor cities: Generally, “artificial moats” with water and small elevations were used to protect the edges of towns. If they had more money, they used stockades known as “”. The most important cities were usually on mountains, and only the areas where there were gates were walled. That’s not lacking in technology, it’s taking advantage of the terrain to your advantage.
Finally, there are at least 2 examples of exaggeratedly walled cities in Japan, in plain areas where the mountains could not be used: One of them was the city of Kamakura, of the Kamakura shogunate. Because by 1380 the city ceased to be the capital, many of the stones of the walls were used for other things, but the elevations remained. The other is in Kyoto, and was known as Odoi, and is estimated to be 22.5 km long. Due to the increase in the size of the city, most of the wall was lost, but its artificial elevation remained, in certain areas:

4).- “DID THEY EVEN BUILD A GREAT LONG WALL:”
Yes, the Genkō Bōrui, but it was not that high (2 to 3 meters), but it was long (20 kilometers), and it was intended to stop the Mongol cavalry from an expected invasion of the west coast. The long-awaited invasion lasted only a few days, since the Mongolian army was again destroyed by a hurricane (how lucky), so the wall was only used very little.
On the other hand, it must be commented, this wall “is underground” because Japan is an archipelago of volcanic islands, every year a layer of earth makes the surface level higher. Many old buildings are underground unless they were elevated or maintained. The parts of the wall in public view are usually “unearthed” areas. Also, during the Edo period (1615-1870) it was decided to use the carved stones from many areas of the wall to build other buildings or fortresses. Finally, the most probable schematization is this:
But it looks like this unearthed, and with many stones removed:
Photo: Imazu Stone Fortifications,Restored(left photo) and Raw(right photo)
5).- WHY DIDN’T THE JAPANESE DEVELOP MORTAR AND LIME IN THE MIDDLE AGES?
Although the Japanese nowadays know how to use cement and mortar, it is something very recent, and here I leave a quote from wikipedia (https://ja-m-wikipedia-org.translate.goog/wiki/モルタル?_x_tr_sl=ja&_x_tr_tl= es&_x_tr_hl=es&_x_tr_pto=sc):
“Mortar came into wide use in Japan after the 1923 Great Kanto Earthquake. Through the Great Kanto Earthquake, the method of building mortar lath walls gradually spread, and during World War II, it was also used carried out fire prevention repairs using batten mortar for the purpose of fire prevention However, even this was of little use, and the result was a burned field, and the need for fireproof performance was strongly recognized in wooden houses, which was prescribed and spread nationwide under this legal standard.”
It must be considered that although weapons and agricultural technology or livestock can be transported to Japan and shown to merchants and scholars, architecture during much of the Middle Ages was an art that was considered “technical”, and not engineering. . It was considered a tradition, even family, or guild, and therefore the construction techniques were sometimes a secret of each country.
An architect could only improve if he traveled to another country and admired its wonders, and then returned to his own with the technique. But that did not happen in Japan, which was also one of the most isolated countries in the world and difficult to access. Consider above that after the Sengoku period (1470-1615), for 250 years, the country was isolated from foreign knowledge, except that which the Dutch brought to a small minority of dignitaries of the Tokugawa shogunate.
In addition, the shogunate itself prohibited any technological research that could be a danger to it. Only after the Meiji Restoration in 1868 were they able to acquire foreign technology, and not all of it.
6.- ABOUT THE DESIGN SEEN IN THE TEASER.-
Of course, aesthetically they should not be straight, but trapezoidal, so I suppose that the walls of the teaser are a Placeholder, just like the spearmen with Heavy Cavalry Lances. But I could be wrong. What should happen is that we should ask for them to be trapezoidal or sloped, like in Total War Shogun 2.