The Boss Door is a placeable Level Object in the boss item category. The door acts as a one-way gate, preventing the player from returning the way they came, as it only opens in one direction. Curiously, only the side of the boss door that the player cannot enter through is considered a solid object. Crossing one resets enemies and level objects to their original positions.
Horizontal doors are entered by simply walking through them, while vertical doors are entered by falling/jumping through it, or climbing a ladder into it.
In versions prior to 1.5.0, the Boss Door could be crossed if there was a Boss or a miniboss. However, those versions were left unavailable for backwards compatibility, while posterior versions block the access to the other side while there is at least one boss active or if there is a miniboss enemy visible alongside the boss door. This was done to adapt the Boss Doors to the concept of multiple bosses introduced in that update. As "minibosses", the enemies included are Hot Dog, Tama and Whopper. If one or more of these enemies are in a screen connected to one with a boss door but too distant to be in the same line of sight, the Boss Door can be used.
- The Boss Door can easily be used to enter an 'unused' space in your level. This can be done by simply placing the door next to (vertical door) or above/below (horizontal door) an unused space, causing the player to be transported through the door into the unused area.
- In an 'unused' area the player can sometimes clip through vertical walls. If there are tiles at the top of the section under the unused section, the player will stand 3 tiles above the actual position of these tiles.
- Boss Doors previous to 1.5.0 can be crossed if there was a boss despite it betraying their purpose.