Mushroom Kingdom's flag.
It was a normal kingdom,until the King of the Koopas Bowser kidnapped the princess. So,a normal plumber called Mario saved the princess,many and many times. The first appearance of the Mushroom Kingdom was in Super Mario Bros.
There are many castles and towns in the Mushroom Kingdom: Peach's Castle, Toad Town, Mario House, Mushroom City, etc.