This is a special dungeon that opens up at the end of the fifth chapter.
In order to help Sakura, who has been infected by the Melt Virus, the protagonist uses the codecast 10,000 Lust Bondage to dive into Sakura’s cyber body.
This dungeon is essentially Sakura’s inner space. Diving into someone to take medical action is a well known science fiction gimmick of old.
The earliest key concept for CCC was “Diving into the minds of the heroines.” So even though the concept changed, this event was left over.
Part of the scenario was cut during development. Originally there was going to be a Heroine Dungeon for Sakura, Rin, and Rani, and Meltlilith would attack the one who had the most affection for the protagonist.
But then Rin and Rani were sacrifi—err, were promoted to be early game bosses, so Sakura was the only one left in the game with a Heroine Dungeon.
Frankly this was kind of implemented in the place of the erotic scenes you can experience in Fate/stay night. You build a deep relationship with the heroine in an RPG styled way.
Just think of it as playing cyber doctor, okay?
But when Takeshi Takeuchi (our BIG BOSS) heard about this he said,
“I see. That’s a good idea. ——Of course, there’s one for Red Saber as well, right?”
Certainly, having a Heroine Dungeon for only Sakura would let down the expectations of the players, so the writers thought about what they could do, and adjusted the scenario so that Servant Dungeons would also be implemented. As always, our BOSS has great intuition for things like this.