From the Super Mario Wiki
The Keymaster is the ninth course in the Story Mode of Super Mario Maker 2. It is set in the ghost house theme and in the Super Mario World style. It is posted by Agent 2, and has a difficulty rating of 3/4 stars.
The Warp Pipe marked by Rotating Blocks leads to an area with three groups of Rotating Blocks, with the Key inside one of the Rotating Blocks on the right, which in turn can only be hit by activating the P Switch.
The Warp Pipe marked by ? Blocks leads to an area with two groups of Boo Buddies orbiting a ? Block and a Hidden Block, respectively, with the Key inside the Hidden Block, which in turn can only be hit by throwing the POW Block at it.
The Warp Pipe marked by Note Blocks leads to an area with a single Note Block and a Trampoline, with the Key above a bridge at the top of the room, which in turn can only be accessed by placing the Trampolines from both the main room and the aforementioned room on top of the Note Block.
The Warp Pipe marked by Munchers leads to an area with bridges featuring Big Munchers atop them, with the Key above a bridge at the top of the room, which in turn can only be accessed by throwing the POW Block so that every Big Muncher is defeated.
The Warp Pipe marked by Twisters leads to an area with a single Twister, with the Key above some Hard Blocks at the top of the room, which in turn can only be accessed by placing the POW Block so that the Twister can move to the bottom right corner and allow Mario to access the bridge at the top of the room.
Additionally, Mario can also take the Trampoline with him back to the starting room to enter the Warp Pipe that spawns Super Mushrooms, allowing him to enter a room with a Koopa Clown Car stationed out of reach. He can then throw a POW Block that spawns out of the Warp Pipe into the Koopa Clown Car so that it comes down and allows Mario to reach another Key.
After at least four Keys are collected, Mario must activate the P Switch so that he can collect one final Key and enter the series of Key Doors from the starting room leading straight to the Giant Gate. Alternatively, he can enter a Warp Pipe to enter another series of Key Doors leading to some coins, then take a Warp Pipe back to the Giant Gate.
"Hey, so I made a Ghost House. I'll pay anyone who can make it to the end. Here's a little hint: you'll need four Keys. ...More if you want to get the true ending. *wink*"
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