This chapter features a complete walkthrough for the mission The Defiled Castle. Here you will find a solution to the challenges of the main plot of the story. It will help you in completing the mission in best way possible. This mission ends with a boss fight with Okatsu.
Details are described in specific sections.
Sewers and courtyard
Before starting the mission you should gather as much antidote as possible. The mission starts near a shrine. Go down to the sewers. Watch out for the mudman opponents. Take down the archer and keep going. Destroy the barrels on your way. You will fight a warrior. Then enter the room and climb the ladder. Destroy the crystal. Enter the room where the crystal was, destroy the barrels and use the narrow passage and stairs to reach the next area. Defeat the skeletons and take the Kodama.
Jump down into the sewers. Watch out for mudmen. By going forward you will unlock a shortcut to the shrine. When you see a large chamber with fog at its end, you can choose from two paths. You can either directly fight yoki from the fog or you can bypass the opponent. For the latter you must climb the stairs on the left. On the way you will encounter an archer and a possessed samurai. Each path will lead you to the crystal that you must destroy. After destroying the crystal you should walk down the stairs and send away Kodama. Then go to the other side of the wall and climb the ladder.
Activate the shrine after getting up and enter the courtyard. The area is patrolled by a few samurais. Actually you need to defeat only one of them - the one with the cannon, on a wooden construction. In order to reach him you must walk to the other side of the area and climb the ladder. Reach the crystal after defeating him and destroy it. Search the lower courtyard for treasures. Then climb the stairs to the higher level. In front of the stairs you will see chests - after destroying them you will find a Kodama.
Defeat the yoki on the field. Don't walk through the door on the left yet. If you do, you will encounter a "boss". Turn behind the corner and climb the ladder. You will encounter a dweller and a Kodama. Below, in the fog, there is an onyudo. Defeat him, collect the items and then enter the arena with the "boss". It's not a regular battle - if you destroy the three crystals you will remove the dark spell from him.
Once you leave the arena and enter the building, turn right and go down the ladder. You will find a door that open shortcut to the shrine. Next to them there is a samurai who will teach you gestures in exchange for dung balls. Go back up. Don't enter the room with samurais. Instead, bypass them through the corridor on the left. Behind the stairs you will find an entrance to a room with a crystal. Destroy the crystal.
Go towards the next crystal. You will see it behind the wall after you climb the stairs. Bypass the room with the samurais and go towards the long corridor. Walk through it while defeating the enemies on your way. You will reach a room guarded by a dweller and a yoki. Destroy the crystal after killing them. Then climb the ladder and destroy the Nurikabe. You will unlock access to a shrine.
Climb the ladder after activating the shrine. Go to the balcony in order to send away Kodama. Inside the building you should open the door behind which you will face the boss, Okatsu.