Unlock Cardboard Characters
     Beat the game on Easy difficulty.

Unlock Characters in Arcade Mode
     Earn  a  Gold  medal  in  "Adios  Amigos" (Arcade Mode) to unlock
Hector Babasco and Lean Molly.

Unlock Characters in Story Mode
     Beat  the  levels  shown below on Medium difficulty to unlock the
corresponding characters:

2280 Return to Planet X: Ozor Mox
2019 NeoTokyo: Sadako
1853 Wild West: The Colonel
1972 Atom Smasher: Khallos
1920 Aztec Ruins: Stone Golem

Unlock Rotating Heads Cheat
     Beat "Pane in the Neck" in Silver (Challenge Mode).

Unlock Streets Arcade Level
     Beat the game on Easy difficulty.