(´・ω・`)「Im Japanesu」
I'm writing with Google Translate.

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It was a Japanese castle that was under construction in Murkwater Construction Site!?

When I saw Murkwataer, remembered Oshi-Castle in Japan. Because, The castle was built on an island a swamp, river and paddy field, and had a similar terrain to Murkwater.
So I built a castle modeled after Oshi castle.
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Building description

Battle of Oshi castle(1590)
Toyotomi clan(20,000 samurais) VS Narita clan(2,000 samurais)
Oshi castle was the residence Narita clan.

In 1590, the Toyotomi clan who unified the west attacked the Hojo clan, which was the largest force in the Kanto region. Toyotomi army also attacked all Hojo's allies. Narita clan was alliance Hojo clan.
Narita clan was a small clan. The difference with the more than 10 times. In addition, the lord(Daimyou) Ujinaga Narita went to Hojo's reinforcement. But, Oshi castle was a castle protected by swamp and river and was extremely defensive. 
Therefore, Toyotomi army decided to submerge the water-protected Oshi castle. Toyotomi army surrounded Oshi castle a 28km embankment and pound the river water. The inside of embankment became a huge lake.
But only Oshi castle didnt sink. 
On contrary, the heavy rain broke the embankment and the water-attack was unsuccessful. The Toyotomi army called reinforcements, and its said that total number reached 50,000. Then, battle between the samurai began in the mud. 
During that time, Ujinaga's father died in the battle.

It was Ujinaga’s daughter princess Kai who took over the command. She was 19 years old.
She seems to have armed herself into the battle and defeated tow samurai with bow.
And about a month after the start battle, end came......

Hojo clan was defeated.
The reason for the war has disappeared.
Oshi castle became the only remaining castle among the attacked Hojo's side.

Toyotomi clan guaranteed the life of the Narita clan and demanded that the castle be surrendered. But, Kai didnt trust and contineued to siege. At the end, her father Ujinaga persuaded Oshi castle to open.

The castle tower is built with this blueprint. 
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-575 Items
-15 beds
-30 water
-10 over food
-100 Electricity
-100 Defense
-Bench work: Weapon, Armor, Chemistry
-4 shops
●Navimesh can only reach upstairs.
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Requirement MODs

↑ Open the "Requirement"!
You need to use all these mods.
I also wrote which mods was used for what. Hmm, As far as I remember.
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Installation:

Manual only
Things to do at Murkwater
●Increase the building ceiling more than triple at Console or G2M or Cheat Terminal or etc.
●Remove the ruins. Tree.
File location
NMM
\Fallout 4\Data\F4SE\Plugins\TransferSettlements\blueprints\1~100(Choose one unused number)\.json file
MO2
\Modding\MO2\overwrite\F4SE\plugins\TransferSettlements\blueprints\1~100(Choose one unused number)\.json file
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enjoy!