Story-Related: Can't be missed. Unlocks in the loading screen for the following stage: Land of Fire-and-Brimstone.
Behind the Looking Glass is the sixth area of the game.
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Walkthrough Tips: Spoilers ahead
Crazed Clockwork--Nothing particularly difficult with this one if you make liberal use of the ice wand and the jacks (both of which you should have by now). There are a few switch puzzles and whenever you get a cutscene you can be sure that a new path or door has opened-up back where you came from somewhere. When you get to the tabletop, jump on every sugar-cube floating in the teacups (make use of the bouncy seat-cushion to do so).
About Face--Boss time! The Mad Matter is an incredibly easy boss on Normal Difficulty--use the jacks and circle-strafe him, staying at midrange so he uses his easily-dodged projectile attack (if you get too close it'll hurt, and if you get too far, likewise). Eventually he will leave and you get to dispatch two robots in any way you deem fit. Their health and manna drops will probably fully recharge you for the second part of the battle which occurs exactly as the first part.
DONE