Where can I find all the Retro Challenges?
There is apparently a challenge which involves finding 25 Retro Challenges (and beating them all too!)
http://www.oxmonline.com/files/u9/runner2-retro1.jpg
Which levels have them, and where can I find them (do I need keys to get there?)
I've found three so far, one on Windy Interlude, one in Cloud 8 1/2, and one in Cloud Nein, but I can't seem to find the rest.
Where are these retro challenges located?
Here's a Steam guide that explains most of it:
http://steamcommunity.com/sharedfiles/filedetails/?id=129947407