Head toward where the Bowser medallion fits. On the way, you may encounter Ant Troopers. They'll lose their spiky helmets after enough damage. When you reach the gate, check it out. Mario places the medallion and the gate lowers, revealing a dirt path up the wall. Use the Trio Drill to climb the wall. There are conveyor belts ahead. If you need to go in the opposite direction of the belt, dash and jump with (X). Go east through the pipe.
The bros wind up in a different part of the Twinsy Tropics. There's a pipe here, encased in some sort of mineral. There's nothing you can do to it for now, so check out the other parts of the area. The pipe to the left leads back into the dungeon. There are spider webs here, but you can dash through them. Hit the ! Block to start a timer. You have 38 seconds to reach the other ! Block. There are many webs in the way. Dash through the least complex web to make good time. If you look below, you'll see the same mineral that's covering the pipe outside. Grab the beans and coins in the wall if you'd like, then walk west. Hit the button with Trio Hammer and the shell on a statue opens. It shoots a beam of energy to the mineral encasing the pipe outside. There's another pipe in the middle. There are Paper Swoops here. Climb the wall and Drill Dash to the rings. Some may rotate to allow you to change direction. Go to the upper right area to find the next button. The pipe to the right leads to an area with more Paper Swoops. The dirt patch is to the far right. The rings here move, so time your Drill Dash. You'll loop around to the upper right, where the last button is.
The last beam of energy destroys the mineral around the pipe. The pipe leads back into the dungeon. Go as far right as you can and the bros will stop. Starlow asks why. Mario has a strange feeling about what lies ahead. Starlow's not too worried about it. Saving may be a good idea. Walk towards the pipe and Starlow says Mario's hesitation was unjustified. Go into the pipe and a Koopaling demands a pass. Another one says Bowser's Castle is through this pipe. Luigi gets knocked out of the pipe. Starlow says his mustache fell off and he should stick it back on. The bros take a few hits and Starlow gets upset enough to use her secret attack: Supernova Sparkle. Unfortunately, the attack hit the bros as well. Larry Koopa appears from the pipe. Starlow thinks Larry's on the small side. This upsets Larry. Luigi's ready to battle, but he's standing next to Larry, who tells him to get on the right side of the face-off. He calls in Ludwig von Koopa to help out. Ludwig got stuck in the pipe.
At the start of the battle, Ludwig wonders why the bros get to use Battle Cards and they don't. He pulls out a card to increase speed, which requires 2 dark Star Points to use. They don't have any at the moment, but will earn them for each time they hit the bros. Ludwig and Larry will attack the bros with their shells, then slowly go past Paper Mario. Hold the jump for as long as possible. Ludwig will attack Mario or Luigi with magic. Counter with the hammer. Larry will attack Paper Mario with magic as well. Jump to dodge, land, then jump again as Larry will charge. Hold the jump for as long as possible as Larry is still attacking. Land to counter. The Koopalings will summon some Shy Guys carrying a giant bomb. Use the Boomerang Dodge. This is another one of those fights where it's easiest to just leave Paper Mario alive. Only Larry attacks. You can revive the bros towards the end of the fight. Once Larry is defeated, Ludwig will attack with a large ball of magic. Counter with the hammer, as before. Once they're defeated, Ludwig concedes the bros aren't as wimpy as they look. Starlow says they'll just pass through the pipe. Larry says them losing was part of the plan. Starlow suggests proceeding with caution.
Go through the pipe. There are conveyor belts carrying ore into Bowser's castle. Minions go back inside the castle. Starlow reminds them that saving the princesses is their main priority. Go through the other pipe. There are Boomerang Bros here. Jump with (X) to cross to stones floating in the lava. The second one will toss the bros high in the air before landing far away. Paper Mario folds into an airplane to save them. The bros want to try something light that. The bros learn Trio Glider. Press (A), (B), and (Y). Press (Y) again up to three times to fly higher. Starlow says it can be used to reach high places as well. Follow the path and go west. A Koopa complains that he can't keep hauling stuff around. Another one agrees with him. Kamek announces that break time's over, so they try to look busy. He says they need to get back to hauling papercrafts.
Go west. Use the Trio Glide to cross the lava. The path splits ahead. Take the path due east, then go east again and hit the ! Block to connect a path back to the pipe. Return west to reach the pipe the Koopas went through.