The Blue Switch Palace is a level seen in Super Mario World. It's unlocked by finding a secret exit in Forest of Illusion 2. After it's completion; all the Blue ! Blocks will be activated, and the level cannot be entered again.
The player needs to switch the gray P switch first and then the blue one. They then need to collect all the gray coins before the gray P switch time expires. The player can then collect the gold coins if they want. Then they need to go through the Warp Pipe and press the big Blue Switch. Which will then activate all the Blue ! Blocks.