World 5 (New Super Mario Bros. 2)
From the Super Mario Wiki
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World 5 is the fifth normal world that appears in New Super Mario Bros. 2. This world is located on a single large cloud, with several tall pieces of land jutting out from below the cloud. The levels here are mainly aerial levels, although some do take place on land, which are mainly cliffs or slopes. The map music is the same one played in World 7 of New Super Mario Bros. Wii. This world can be accessed by completing World 4-Castle or World Flower-Castle. This is also an optional world as it can be bypassed by completing World Flower-Cannon instead, leading to World 6.
There are a total of 11 levels and four Toad Houses that can be found here. The 11 levels include eight levels on the main path (including the Fortress and Castle), two secret levels and the rainbow course. One of the secret levels is the world's Ghost House, which is at first obscured by clouds and can only be seen on the map when it is unlocked. Of the four Toad Houses, two are red and two are green. The boss of this world is Ludwig von Koopa, who is fought at the castle. Completing this world will open up World 6.