Mogeko Castle is the first game made by Mogeko (site name Deep-Sea Prisoner) in RPG Maker. It was first released in April 1st, 2012 in RPG Maker 2000. A remake made in RPG Maker VX was released on early 2014.
Like Mogeko's third game, Wadanohara and the Great Blue Sea, this is rated by Mogeko themselves as for being for age 15+, but this a Japanese standard of rating and depending on a country's standards, age 18+ might be more appropriate. This game contains very violent imagery, and implied sexual assault. Unlike Wadanohara and the Great Blue Sea, these elements are apparent from the start of the game.
The game starts off a Mogeko, a cat-like yellow being, watching over someone (who is presumed to be another Mogeko at first) and decides to tell them a story. From there, the game officially begins.
Yonaka Kurai is an ordinary high school student, sitting in a train waiting to get home in order to see her brother, Shinya, who has been gone for some time. As it seems it'll be a long train ride, you are given a choice to go sleep (you still go to sleep), and Yonaka wakes up to arrive in a station called "Mogeko". Confused, she gets off of the train and walks forward. Throughout the train station, posters (one says "I WANT GYOZA" written in blood) are posted over the fence, which has some holes. Yonaka reaches a forest and encounters a group of Mogeko, who say that they'll be "gentle". You have to run until you reach a castle, and must enter unless you want to see a bad ending.
Throughout the course of the game, your goal becomes apparent; you must reach the top floor in order to get back home. In one of the floors, you are thrown into a prison cell while a group of Mogeko decide what to do with you. The prison cell you are thrown already houses a certain Mogeko who will help you through the game.
The game has multiple bad endings, a normal end, and the true end. This game is meant to be a satire on otaku who say things like "my waifu" and the like, often found posting on certain areas of 2ch.