After unlocking the red ! boxes, go to the castle courtyard. Punch the giant boo with the cage inside of him to enter Big Boo's Haunt. Obtain the first star by defeating all the Boos to get the staircase. In the second goal, as Mario, go up the stars and into the room with the single bookshelf. Use the fire flower to float up to the room above, where you'll find a Luigi painting. Jump through the painting and defeat Big Boo to get Luigi's key.

Submitted by - y_a_link

Once you obtain 8 stars, enter the rec room and unlock the 8 star door. Jump into the Mario picture and defeat Goomboss to obtain the key for Mario's Room.

Submitted by - y_a_link

After obtaining 14 stars, stand on the Sun carpet in the main plaza. Enter first person view mood and look up to the beam of light. In the new world, stomp on the red switch in the middle tower to unlock the red ! boxes.

Submitted by - y_a_link

After obtaining 30 stars and defeating Bowser for a 2nd time, head up to the second castle floor and enter the room where Snowman's land. On the left you'll see a Luigi painting, but its reflection in the mirror will be a Wario painting. Get the flower to turn invisible and walk through the mirror to appear on the opposite side. Jump into the Wario painting to enter the level. Defeat Chief Chilly to obtain the key for Wario's Room.

Submitted by - y_a_link