Mushroom Kingdom II
Mushroom Kingdom II (キノコ王国 II, Kinoko Ōkoku II) is an unlockable stage in Super Smash Bros. Melee. It is unlocked by obtaining the Birdo trophy. Luigi is fought when unlocked here, even if the Birdo trophy has not been obtained. It is Luigi's home stage, mostly, but Mario and Peach are also natives. In All-Star mode, this stage is played on when the player faces Dr. Mario and any of his teammates.
Contrary to what its name suggests, this stage is derived from Super Mario Bros. 2's Subcon area (specifically the version seen in the 16-bit remakes of SMB2), rather than the Mushroom Kingdom itself. It is made up of 3 sections. There are two upper platforms on either side of the stage and one lower one in the middle. The center platform is actually 2 platforms connected by a log. There is a pit on either side of the middle platform.
Logs come from the waterfall and fall into the pit. One can jump on them, but they fall rapidly. On occasion, Birdo will appear from the side of the screen and lob eggs into the fray. The eggs can be broken and sometimes have items in them. Birdo can be knocked out, but she will come back. Pidgit will fly over the center on a carpet, which one can jump on.
In 1986, Super Mario Bros. 2 (known as Super Mario Bros.: The Lost Levels outside of Japan) was released in Japan. However, it was never initially released outside of Japan due to Nintendo of America's dislike of the game's difficulty and similarity to Super Mario Bros. rather than being a true sequel. Rather than risking the popularity of the Mario series, Nintendo decided to look for an alternative game. The game that was ultimately chosen was the Japanese game Yume Kojo: Doki Doki Panic. Nintendo replaced the playable characters in Doki Doki Panic with Mario, Luigi, Peach, and Toad, and released the game as Super Mario Bros. 2 for the NES outside of Japan. Other than the playable characters, the two games are almost identical.
This stage is based around the Super Mario Bros. 2 that was built around Doki Doki Panic. In the ending cut-scene it is revealed that the entire game took place in a dream Mario had. Also, the world that Super Mario Bros. 2 takes place in is not Mushroom Kingdom, but instead Subcon. Despite these differences the stage is called "Mushroom Kingdom II". However, the message that pops up when unlocking the stage says,"Mushroom Kingdom II is open; it's the dream world, Subcon!"
The majority of Mushroom Kingdom II is based around World 1-1 of Super Mario Bros. 2. In World 1-1, there is a point where the player must jump on logs in order to complete the level. The logs in Melee also can be used as platforms as they go down the waterfall. Mushroom Kingdom II also features two enemies that appeared in Super Mario Bros. 2: Pidgit and Birdo. A Pidget is a small bird that controls a magic carpet and tries to attack the player. The player can defeat Pidgets just by simply jumping on it and throwing it off the carpet, which the player can now control. A Birdo is a boss that appears in the last level of every World. Its main method of attack is to shoot eggs at the player.
When Super Mario All-Stars was released for the SNES, it contained Super Mario Bros. 2 and three other Mario games with updated 16-bit graphics. In Melee, the design of the stage is based around the 16-bit version rather than the original 8-bit version. 
The music that plays on this stage is the orginal 8-bit version of the track that played in all outside stages of Super Mario Bros. 2. When the time runs low in Melee, the track is changed to Super Mario Bros. 2's boss theme.