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I keep seeing mention of this on Youtube; custom levels, and a level editor, but I don't see it in the game menu. Is it separate, or how is it accessed?
This question / problem has been solved by VanishedOneimage
I know this is an old thread, but in case anyone else wanted the answer: 'It's heavily linked to the Steam Workshop API.' Via https://docs.google.com/spreadsheets/d/1zjwUN1mtJdCkgtTDRB2IoFp7PP41fraY-oFNY00fEkI/edit#gid=0