World 6 (Super Mario 3D World)
From the Super Mario Wiki
World 6 is the sixth world of Super Mario 3D World. It is a group of clouds floating near World 5 and World 1; in fact, World 5 is still visible near the start of the world. However, only the Castle and Enemy Battle are shown. ? Blocks and Coins are scattered throughout the map. It contains seven standard courses, two Enemy Battles, a Toad House, a Lucky House, a Mystery House, a Castle, a Boss Arena, and a Sprixie House. After completing the castle, the cyan Sprixie Princess will create a Clear Pipe to Motley Bossblob's Big Battle, and completing that level unlocks World Castle.