From the Super Mario Wiki
Mario's Castle appears in Super Mario Land 2: 6 Golden Coins. Wario steals the castle and transforms it into a level Mario must proceed to reach Wario.
Inside are metal spiked balls, Piranha Plant statues that spit fire, and traps shaped like Wario's gloves known as Gen Kottsu that try to crush Mario. Extremely odd creatures live here such as Don Gabamen: a type of bouncing ball with Wario's face on it. If Mario beats Wario, the spell is broken and the castle turns white instead of black (and its "M", turned upside down to resemble a "W", is moved back into its correct position).
In later games, Mario's Pad replaced the castle as his place of residence in the Mushroom Kingdom.
In Mario Party 8 on Koopa's Tycoon Town, if the player gets a three-Star hotel after trading coins with a Koopa Troopa at a hotel, the hotel will look like Mario's Castle.
Stage layout of Mario's Castle after it was taken over by Wario