Bowser Gate

From the Super Mario Wiki

Bowser Gates are huge stone faces of Bowser that act as doors in Paper Mario. In three areas of Bowser's Castle a Bowser Gate 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 commentary is hilarious for most of them.

The first one Mario comes into contact with will open a trap door, sending Mario to a jail cell, after Mario escapes the gate lets him in. The second will quiz Mario five times on various enemies, however, if Mario's wrong three times he'll have to fight three Anti Guys. The third and final is a battle with the new and improved Koopa Bros., who get knocked away by the improved Jr. Troopa, whom Mario will have to fight. The third Bowser Gate is the exit from Bowser's Castle, leading to Peach's Castle.