Yellow Switch Palace
From the Super Mario Wiki
The Yellow Switch Palace is a level in the game Super Mario World that contains a giant, yellow ! Switch. It's the first Switch Palace that Mario and Luigi find, as the palace is not hidden. It's on top of Kappa Mountain in the northwest of Yoshi's Island, and is accessed by completing Yoshi's Island 1. The entire level contains no enemies or powerups and cannot be replayed after it has been completed.
The palace consists of two rooms, which the player has 200 seconds on the timer to complete. The first room of the Palace is a large open area with a single P-Switch. When the switch is hit, the room will get filled with about 500 Coins, and the player will have about ten seconds to collect as many as possible; however, the player is not required to hit the P-Switch to progress through the level.
A Warp Pipe will guide the player to the second room, which contains the yellow ! Switch. The player can simply jump on top of it to activate it and end the level.
 Gameplay effects
Once the yellow ! Switch is activated, all the yellow Dotted Line Blocks will turn into yellow ! Blocks. During gameplay, a Super Mushroom will appear when a yellow ! Block is hit. These blocks also help the player progress through levels by filling in some gaps. The Yellow ! Blocks only appear in 20 levels after that.
 Levels With Yellow ! Blocks
Yellow ! Blocks can be found in these levels: