The Dev Apartments are found by travelling to where the Mysterious Door would normally be in Undertale. Instead, you will arrive in a corridor with a similar tileset to The Caverns, though everything has been tinted yellow. Travelling further towards the top of the screen will reveal a counter at which statues are lined up, as well as an out-of-order elevator and a staircase off to the right that leads to three seperate floors of locked doors. Each statue corresponds to a developer, and a locked door will also be linked to that developer, but not in order unlike the statues. The last door on the third floor can be entered, but requires the Gemini Constellation Card. While in the main room of Dev Apartments or any one of its floors, the music track called "宇宙ホテル" (Japanese for "space hotel") will play.


The last door on the third floor is entirely red apart from the door handle and keycard slot, showing no pattern unlike the other wooden doors. Attempting to open the door simply reads "Locked.", unless you have the Gemini card, upon which the door will read "You used the Gemini card."

Upon entering the Gemini room, the music changes to "room_yourfault", and your player sprite is replaced with a greyscale version that is missing its face. The room is a long, winding road that shares its aesthetic with OFF's Zone 0. Jelly objects that appear to be keyboards wander around the room, and attempting to talk with them will return a blank dialogue box. At the end of the road is a throne with what appears to be a foetus on it. Approaching it will enter an un-winnable battle which does not change the music.

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