Mogeko Castle is a "prosciutto adventure game" made by Deep-Sea Prisoner in RPG Maker VX Ace. It's not an RPG - there are no battles, and the story is totally linear.
Yonaka Kurai, an extremely ordinary high schooler. She takes the usual train home.
Though it's all ordinary, today is somewhat special. Today, her beloved brother comes home.
But after waking from a nap, the train has stopped at an unfamiliar station...
Deep-Sea Prisoner rates this game "ages 15 and up." (This rating is Japan-centric and would likely be more strict by other countries' standards; 18 and up may be more appropriate.)
It is very violent and contains instances of implied sexual assault, with the bad endings being some of the most severe parts. It also tends to treat serious topics (mainly sexual assault) like jokes, so I would strongly advise not playing if you think you may be upset by such content.
I do not support everything portrayed here. Deep-Sea Prisoner also insists players should "keep reality and fiction separate."
(Content Severity: Severe)
Click here for content warnings.
Extreme violence, especially in bad endings.
Frequent references to sexual assault by the Mogekos.
Murder and death.
(Chrome may falsely flag the download as malicious, but it's safe to ignore.)
Installation of the RPG Maker VX Ace Runtime Package (not VX) is required to play.
Download VX Ace Runtime Package (RTP)
The game's engine is made for Windows only, with no native support for other platforms. To play on Mac, you can try a general-purpose tool like WineBottler.
Playthrough videos of Deep-Sea Prisoner's works are now allowed. However, they ask to refrain from monetization of any form, due to the terms of the free resources used.
 Japanese version readme
Make sure to extract the game to its own folder instead of running directly from the ZIP file. If you don't do so, any saves made will be lost the next time you run the game.
If you extracted the ZIP but save data is still being erased, try running the game as an administrator. Without admin privileges, the game may not have permission to create save files.
"RPG Maker VX Ace RTP is not installed" means you need the RPG Maker VX Ace RTP. This is distinct from the VX RTP which other games may require. The download link is up above.
Some have experienced crashes during the final chase scene in Chapter 6. There are many objects in that map, so it most likely occurs because lower-end computers aren't able to handle it. If it consistently crashes, download this save, copy it into the SAVE DATA folder, and load slot 20 to skip to after the chase.
Though the game is quite straightforward, there are two parts in particular which require some trial and error. So I figured I'd be nice and give the answers here, if you want to "spoil" yourself.
Middle, left, right, middle, middle.
Chase to Floor Seven
Keep going up at the first split, then up at the second, then right at the third.
Keep going until the next split and go right, then open the door at the end of the passage.
Go down at the split, and keep going to the door at the end.
Go down at the split, then quickly go through the door in the open area. Lastly, just keep going up.
Also, there are some things you might miss. (Spoiler warning!)
Bad End 0
At the front door of the castle, choose to look for another way.
Bad End 1
Check the crazed Mogeko along the dark path on Floor 1.
Bad End 2
Enter the top-right door on the Bowel Path on Floor 2.
Bad End 3
Push the button to open the big cage on Floor 3.
Bad End 4
After Hasu's ascension on Floor 4, backtrack from Moge-ko's room.
Bad End 5
Touch the Blancfluff Mogeparasite in one of the rooms on Floor 5.
Bad End? 6
Outside the library, elope with Defect Mogeko.
Bad End 7
Get caught by Shinya.
Kill at least one of the special Mogekos.
The Door of Truth will present you with the Path of Paradise.
Either don't kill any of the special Mogekos, or continue after the normal end, and choose the Path of Falsehood.
(These contain spoilers for all aspects of the game. Only read after you've beaten it fully.)