Unlock Mario Key:
Defeat Goomboss behind 8-Star Door in the Rec-Room
Unlock Luigi Key:
Defeat King Boo in Luigi's Painting on the 3rd Floor of Big Boo's Haunt
Unlock Wario Key:
With Luigi in the Mirror Room, hop into Wario's Painting and Defeat the Giant Ice Bully
Unlock White Room Key:
Catch all the Shining Rabbits
Unlock Cannon:
To access the cannon outside the castle, get 150 stars. There are three 1-ups, a cap and Luigi's rabbit.
Defeat Goomboss behind 8-Star Door in the Rec-Room
Unlock Luigi Key:
Defeat King Boo in Luigi's Painting on the 3rd Floor of Big Boo's Haunt
Unlock Wario Key:
With Luigi in the Mirror Room, hop into Wario's Painting and Defeat the Giant Ice Bully
Unlock White Room Key:
Catch all the Shining Rabbits
Unlock Cannon:
To access the cannon outside the castle, get 150 stars. There are three 1-ups, a cap and Luigi's rabbit.