Yellow Switch Palace

From the Super Mario Wiki

The Yellow Switch Palace is a place containing a giant, yellow ! Switch. It's the first Switch Palace that Mario and Luigi find in the game Super Mario World as the palace is not hidden. It's on top of Kappa Mountain in the northwest of Yoshi's Island.

The first room of the Palace contains a P-Switch. When hit, the whole room gets filled with Coins and the player has about ten seconds to collect as many as possible. A Warp Pipe guides the player to the actual ! Switch. When it is activated, all the yellow Dotted Line Blocks turn into yellow ! Blocks. During gameplay after the switch has been activated a Super Mushroom appears when hitting a yellow ! Block.

Switch Palaces
Yellow Switch Palace | Green Switch Palace | Red Switch Palace | Blue Switch Palace