With Garro's help, you get past the guards into Nimbus Castle to try to reach Valentina.
After the cutscene, leave the room and use the Save Block. Try to go through the doors and there will be a cutscene. You will automatically run back to the pedestal. Whenever Dodo's head goes back, be ready to jump to avoid getting pecked. Make sure not to move except to jump. The background music also reflects what Dodo is about to do. After your second jump, Dodo will quickly run back to you and peck without much warning, so be prepared. If you get pecked, you get two more chances to try to avoid it, but if you aren't successful, you have to battle Dodo. If you do succeed, you get the Feather, an accessory.
Go into the next room, pick up the Feather if it's there, and save at the Save Block. Then go through the doors. There will be a cutscene.
The Orbison enemies in this area are only weak to Jump attacks.
You can optionally go northwest and fall down below to get a Flower. If you do, then go southwest and follow the path until you are in a room with statues. Then go northeast to return to where the Birdys were hitting you.
Go through the northeast door. Stay near the southeast wall and run northeast to avoid letting the Pinwheel push you back. Then go southeast to the next room.
The Birdy "statue" here will come to life and fly toward you. Hit the chest to get a healing mushroom, then go northeast through the doors. You can optionally go through the eastern door to find a Flower at the end of the path. Then go back to the hallway and go northwest. There is a chest in the northwest corner that contains a Flower.
Go through the northwest door (the door next to the chest that you just opened), then in the room with the two Shamans, go through the northwest door, to reach a room where the path goes southeast. Defeat the Jawful that is blocking the southeast exit, then walk into the wall across from where the Jawful was standing (where it was hitting with its fork). After you have gone as far southwest into the wall as you can go, start going west. When you reach a dead end, jump to reveal a hidden chest that contains a Frog Coin. Then go to the spot where the Jawful was standing, and jump to reveal another hidden chest that contains a Frog Coin.
Go through the southeast exit, then talk to everyone in the room. The person in the west corner will give you Castle Key 1. Another person gives you a Flower Jar. Heal up before you use it, because it will refill your FP. Then save at the Save Block and leave the room.
Go back the way you came until you're back in the hallway with the Heavy Troopa in the middle. Interact with the Heavy Troopa to start the battle. They are weak to Jump attacks, so have Mario jump on them to defeat them. Then check on the door that the Heavy Troopa was guarding to unlock it. Go through. There will be a cutscene. The egg has the key that you need to continue, so check on the egg and agree to play. You will battle Shelly. Just use regular physical attacks to break the shell open. Then you will battle the real boss, Birdo.