The Guard Door is a door that has a stone face of Bowser in Paper Mario. It would appear three times throughout Bowser's Castle as they will require Mario to do something in order for him to pass through them. When this objective is completed, he will be able to progress on with his adventure.
The first door would open a trap door and send Mario into a jail cell. When he escapes, the gate will allow him to pass through. The second door will quiz Mario five times on various enemies that appear in the room. If he answers three incorrectly, he will have to fight three Anti Guys. The last door will have Mario take on the Super Ultra Mighty Koopa Bros. the Great but they will be knocked away by [[Jr. Troopa] and Mario will have to battle him instead. This door will lead Mario out of Bowser's Castle and lead to Peach's Castle.
The Guard Door would be renamed to the Bowser's Gate in Mario Party 9. A Bowser Gate would appear at the end of each board, where the players will have to take on the stage boss in their respective minigame.