From the Super Mario Wiki
Switchblack Ruins, or World Castle-2, is the second course in the seventh world of Super Mario 3D World, World Castle. It becomes accessible by beating Fort Fire Bros., and its own completion unlocks Boiling Blue Bully Belt.
The level starts in a dark hallway that leads to the first set of Switch Panels and a Spiny. Activating the Switch Panels opens up the path to some Question Blocks and the second set of Switch Panels guarded by two Spinies. Stimulating this set of Switch Panels leads to the level's Checkpoint Flag, the third group of Switch Panels and a Fire Piranha Plant. A moving platform takes the player to the final Switch Panel set, guarded by four Spinies and two Fire Piranha Plants. A nearby pipe leads to the Goal Pole.
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