Red Key is an unlockable active item.
- While held, a red outline of a door will appear on walls which a Red Key can be used on.
- You cannot use the item unless you are close to an outline.
- In rooms with multiple outlines, it only opens the closest one.
- A red room can be created through another red room.
- The red rooms have a physical presence on the floor unlike Devil Rooms or I AM ERROR rooms, and can be re-entered at any time.
- Red rooms follow secret room rules, meaning that all other rooms it connects to must have clear space for a door to the red room.
- As a result, all red rooms are symmetrical, and if there's enough empty space you create a large area of red rooms.
- Although a red room can be adjacent to more than one room, if the room type is one that requires something like a key, each entrance to the room will have its own lock.
- You cannot build a red room outside the boundary of the map. Trying to do so will open a door which leads to the I AM ERROR room instead, which you cannot return from.
- You can tell if a red door will lead to an I AM ERROR room if it does not show up on the map.
- The max boundary size of a map is 13x13 rooms.
- The max number of rooms in a single floor is 128, counting all the rooms of the original map (including Secret Room and Super Secret Room, also double and quadruple-sized rooms count as one) plus the rooms created. After the count of 128 rooms the Red Key will not open new doors and the charge is wasted.
- Red rooms can be special rooms even in The Womb / Utero / Corpse, Sheol, Cathedral, Dark Room and The Chest, where these types of rooms otherwise would not be generated.
Bug! After quitting and re-entering the game, using Red Key will cause a no-room-config error until the next floor is reached.
Bug! Created rooms may interfere with the operation of the room-teleporters in Mausoleum, causing the teleporter to lead to the wrong room.