Where can I find all the Retro Challenges?

Question

There is apparently a challenge which involves finding 25 Retro Challenges (and beating them all too!)

enter image description here

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?

Retrosaur · Accepted Answer · 2013-03-02 04:22:05Z

up vote -1 down vote accepted

Here's a Steam guide that explains most of it:

answered Mar 2 at 4:22

Retrosaur
5,22683079

1 Answer