This is a LEGO Indiana Jones walkthrough on how to unlock the secret level.
What you basically need are most of the characters and at least one level in each chapter having 10/10 artifacts. You might not need all of the artifacts to play the Young Indy and Ancient City level, but you will definitely need them to play the last level though.
Just watch this video tutorial to learn how to unlock the secret level on LEGO Indiana Jones. It tells you how to get the key to unlock them.
For a more organized list and even more video game walkthroughs, check out NextGenWalkthroughs.
Lego Indiana Jones: The Original Adventures is a 2008 video game which allows players to recreate moments (albeit more humorously) from the first three Indiana Jones films. It is available for Nintendo DS, Microsoft Windows, Mac OS X, PlayStation 2, PlayStation 3, PlayStation Portable, Wii, and Xbox 360.