The Maskplex is a high-rise hotel with a huge Shy Guy mask on it. Shy Guys from the surrounding area are constantly entering, with two Snifits guarding the entrance. If you enter normally, you will be shot. Only by wearing a Shy Guy mask can you gain entrance. A mask can be obtained by punching a Shy Guy in the back, causing his mask to fly off.
The Maskplex is an ordinary hotel, except that it is filled with security/booby traps. Although it is possible to evade these, they can be easily bypassed simply by keeping your mask on. Attacking a Shy Guy will sound the alarm and trigger a Black Pit Bob-omb detonation. So, getting through this Super Castle relies on patience. The two Iron Goombas are blocking a blank wall, and are wearing masks to pass undetected. You can knock off their masks, and they will flee when the sirens sound. They will break through the wall to reveal an elevator to General Guy.
After General Guy is defeated, a Goombe will run out, making siren sounds. Eviction notices wil appear on every room door, causing those doors to open onto a void. By throwing the Goombe at the wall, you create a hole to the outside world. Mario can then become Balloon Mario and carry the entire group down to earth. You can then catch a ride on the train to the Dark Land, site of Bowser Castle.