Unlock Cardboard Characters
     Beat the game on Easy difficulty.

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

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

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.