After you go through Booster Pass, you arrive at Booster Tower, where you find an old enemy.
Explore the Tower
After the cutscene, check on the door and you find that it is locked. Try to leave the area, and there will be a cutscene, and Bowser will join you. You can optionally get an explanation of how to switch allies. Now there is a "Switch" option in the menu that allows you to change the two characters in the active party with Mario. You might want to keep Mallow in the party for his healing abilities. You can equip Bowser with an Accessory. You can optionally go back to Moleville and buy Work Pants for Bowser. Afterward, go through the door to enter the tower.
In the reception area, if you touch any of the Spookums or the Snifit, you will have to fight. Go northwest and jump up the floating stairs. On the upper walkway, you find family portraits of Booster. You might want to look at each one and make a note of which is which. Then go southeast and then go through the door.
In this room, there is a constant flow of Spookums coming down the stairs. When you reach the northernmost corner, go slightly southwest from the northernmost corner, and jump to find a hidden chest. Then go southwest to the next room.
There will be a cutscene. Afterward, go southeast, then northeast, and before you go through the exit in the southeast wall, go northeast where the tracks are going, and you can find a Flower Tab hidden behind the wall. Then go through the southeast exit.
Make your way through this room full of Rob-ombs, and you will see an exit in the middle of the southeast wall. If you want to ignore it, go to the next paragraph. This leads to a room with a floating chest. If you wall southeast off of the edge of the ledge, you should land on a see-saw, and the Bob-omb on the other side will be launched upward, and when it lands, you will be launched up and you can hit the floating chest to get Masher, a powerful weapon for Mario. Then go southwest to exit the room, and you will be back in the reception area. From there, make your way back to the room full of Rob-ombs.
In the upper part of the room full of Rob-ombs, go to the southwest end of the walkway and go through the westernmost exit in the southeast wall.
Jump on the green switch. This will open a secret cave in Booster Pass (the area that you went through to reach this tower). Then go through the exit in the southeast wall.
Go southeast and up the stairs. You can optionally go behind the curtain to see an easter egg. It will wear off when you try to leave the room. When you're ready, go through the northeast door.
Go northeast and make your way up the stairs with the Spookums. At the top, there is a Snifit that is shooting Bullet Bills. Go through the door near the Snifit.
Go up the stairs and get past the Rob-ombs, and go through the door at the end.
Save at the save block, then go through the southeast door. The note on the wall says to look at the family portraits in order from oldest to youngest. The portraits were also on display in the reception area. If you didn't make a note of their order, you can click the button below to see the order to check the portraits:
If you get it correct, the portrait will flip around and show the letter "B". If you check on a wrong one, a monster will attack.
After you check on all of the correct portraits, you get the Elder Key. Go northwest to the locked door and check on it. Jump up and try to touch the Chomp. There will be a cutscene. You can now equip Chomp as a weapon for Bowser. Leave the room.
In the room with the portraits go southeast and go through the exit at the end.
Make your way upward and you find a Thwomp on a see-saw. Before you use the see-saw, go to the east corner and hit the treasure chest to get a mushroom. Then jump on the other end of the see-saw to be launched upward to the next room.
From where you arrive, go to the west corner and jump to find a hidden chest. Then make a running jump from here to the platform where the Spookum is walking around. Go to the east end and jump onto the large floating block, and make your way upward through the room. There is a Spookum walking back and forth on the narrow walkway next to the flight of stairs, and if you keep going northeast past where the Spookum is, you can jump to find a hidden Frog Coin behind the wall. Go back to the flight of stairs and go up, then go through the door at the top.
In this room, there are many Fireballs hiding in the floor, as well as some coins on the floor that you can collect, and also the Room Key. Get the key, then unlock the door in the northeast wall and go through to find a chest that contains Zoom Shoes. Go back to the previous room and go through the door in the northwest wall.
Get past the Chomps in this room and go through the exit at the end.
There will be a cutscene. When the bombs are floating down, you can go to the east side of this room and hide behind the wall to avoid them. After the cutscene, go through the exit in the southwest wall.
Go northeast up the stairs, then go through the door at the top.
Go north and save at the save block, then go to the southeast part of the room. In the south corner here, jump to find a hidden chest. Then jump up the floating blocks to reach a chest that contains a Frog Coin. If you can get on top of that chest, you can jump to find a hidden chest that contains a Goodie Bag.
When you're ready, go to the southeast part of the room and go through the door in the southwest wall.
You arrive in a room with a cage near the door, and a box of toys. Try to go through the southwest door, and there will be a cutscene. When Booster says "It's behind those curtains! Number one, you go and look," make sure not to be behind the curtains that the Snifit opens. (If you're not sure where you are, you can either jump, or go southeast so you can see your feet below the curtains.) Then a second Snifit will look at the same time. As before, just make sure not to be behind a curtain that a Snifit is about to open. Then the third one will join in, so there will only be one safe curtain to stand behind. After that, there will be a cutscene. When Mario stands beneath the Mario doll, press the Jump button.
You will get an Amulet. You will also learn the password to the door to the balcony: Mario. Go through the southwest door, and you will fight Knife Guy and Grate Guy.