Guard Door

Guard Doors are huge stone faces of Bowser that act as gates in Paper Mario. In three areas of Bowser's Castle a Guard Door can be found, they'll need to have something be done before they let Mario pass. If Mario completes the objective the gate will split in half and open.

The first one Mario encounters opens a trap door, sending Mario to a jail cell. After Mario escapes, the gate lets him in. The second quizes Mario five times on various enemies; however, if Mario answers three questions wrong he is forced to fight three Anti Guys. The third and final Door sends Mario into a battle with the Super Ultra Mighty Koopa Bros. the Great, but they are knocked away by the improved Jr. Troopa, whom Mario then has to fight. The third Guard Door is the exit from Bowser's Castle, leading to Princess Peach's Castle.

Trivia

 * When the 3rd Guard Door at the end of the castle encounters Mario, he claims he has much better defense than the other Guard Doors. If Mario answers three out of five questions wrong during the encounter with the second Guard Door, he forces Mario to fight three Anti Guys, and it seems highly unlikely that the Koopa Bros. would have become significantly stronger than three Anti-Guys (they have 50 HP and can do 10 or 12 damage each turn). However, the Koopa Bros. aren't fought due to the interference of Jr. Troopa, who is easier than three Anti Guys as well.