Infinite Ammo
Complete Story Mode on the Hard difficulty setting to unlock infinite ammunition in all game modes.

Big Tony
Beat the Mafia Mission on normal to unlock the master...
Big Tony

Unlock arcade level called Streets
To unock the arcade level ''Streets'', just beat the game on easy difficulty.

Unlock Cardboard Characters
Beat the game on easy to obtain the Cardboard Characters cheat.

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

Unlock Rotating Heads Cheat
To unlock the rotating heads cheat, just beat Pane in the Neck in Silver (Challenge mode).

Unlock Some Silly Hats
Viking Hat - Beat ''Silent But Deadly'' with a Gold or Silver medal

Pirate Hat - Beat ''Trouble At The Docks'' with a Gold or Silver medal.

Big Ears - Beat ''Escape from Neotokyo'' with a Gold or Silver medal.

Unlock the Site Multiplayer Level
I noticed the level unlock code someone submitted didn't list The Site.
To unlock The Site level, complete Story mode on Hard.

View the Ending Movie
Successfully complete the Space Station on Easy mode to view the ending movie.


Unlock Multiplayer Characters In Story Mode
When you beat a level on normal or hard, a character for arcade mode is unlocked. Here's the list ..

Siberia - Hybrid Mutant
Chicago - Big Tony
Notre Dame - Jacque de la Morte
Return To Planet X - Ozor Mox
NeoTokyo - Sadako
Wild West - The Colonel
Atom Smasher - Khallos
Aztec Ruins - Stone Golem
Robot Factory - Machinist
Space Station - Reaper Splitter


Unlock Multiplayer Maps
Mexican Mission: Available by Default

Ice Station: Available by Default

Hospital: Available by Default

Training Ground: Available by Default

Aztec: Available by Default

Scrapyard: Clear Arcade League - Amateur League - Mode Madness - Scrap Medal with a Bronze Medal or higher

Nightclub: Clear Arcade League - Amateur League - Too Hot to Handle - Disco Inferno with a Broze Medal or higher

Hangar: Clear Arcade League - Amateur League - Beginner's Series - Top Shot with a Bronze Medal or higher

Robot Factory: Clear Arcade League - Amateur League - It's a Blast - Demolition Derby with a Bronze Medal or higher

Ufopia: Clear Arcade League - Honorary League - Maximus - Cold Corpse Caper with a Bronze Medal or higher

Chinese: Clear Arcade League - Honorary Leauge - Elimination Series - Starship Whoopers with a Bronze Medal or higher

Chasm: Clear Arcade League - Honorary League - Outnumbered but Never Outpunned - Can't Handle This with a Bronze Madal or Higher

Streets: Clear Spaceship: 2401 on the Easy difficulty setting

Compund: Clear Spaceship: 2401 on the Normal difficulty setting

Site: Unknown, Probably linked to amount of Arcade League matches or Challenges completed

Circus: Clear Challenge - Monkeying Around - Dam Bursters with a Bronze Medal or higher


Ending bonuses:
Successfully complete the game in story mode under the easy difficulty setting to unlock the Cardboard Characters cheat and the Streets multi-player level in arcade mode.

Successfully complete the game in story mode under the normal difficulty setting to unlock Big Heads and All Characters Cloaked cheats and the Compound multi-player

level in arcade mode.
Successfully complete the game in story mode under the hard difficulty setting to unlock the Unlimited Ammo cheat and the Site multi-player level in arcade mode

Arcade League (Amateur) bonuses:
Beginners Series
Adios Amigos!: Hector Babosal, Lean Molly (Gold)
Casualty: Dr. Peabody, Crypt Zombie (Gold)
Top Shot: Hangar Level, Sgt. Shock, Elimination mode (Gold)
Mode Madness
Chastity Chased: Regeneration mode (Gold)
Shrinking from the Cold: Private Sand, Sgt. Slate, Shrink mode (Gold)
Scrap Metal: Scrapyard, Chassisbot (Gold)
It's A Blast
Night Shift: Sentry bot, Leech mode (Gold)
Spoils of War: Meezor Mox, Thief mode (Gold)
Demolition Derby: Male Trooper, Female Trooper, Robot Factory Level, Vampire Mode (Gold)
Too Hot to Handle
Monkey Immolation: Crispin, Flame tag (Gold)
Disco Inferno: Louie Big Nose, Lt. Wild (Gold)
Burns Department: Undead Priest, Nightclub, Virus mode (Gold)
Team Series A
Club Soda: Slick Tommy, Jimmy Needles (Gold)
Station Stand: Lt. Shade, Zones mode (Gold)
Men in Gray: Accountant, Kawer, Assault mode (Gold)


Arcade League (Honorary) bonuses:
Maximus
Cold Corpse Caper: Cyber Fairy, Gargoyle (Gold)
Killer Queen: Leo Krupps (Gold)
R109 Beta: Ufopia, Gladiator mode (Bronze), Roman Hat, Lt. Chill (Platinum)
Elimination Series
Baking For The Taking: Chinese Chef, Gingerbread Man (Platinum)
Brace Yourself: Braces, Trooper Brown (Gold)
Starship Whoopers: Chinese, Monkey Assistant, Trooper Black (Gold)
Burns 'n Bangs
Chinese Burns: Calamari, Chef Hat (Platinum)
Snow Business: Snowman, Trooper Grey (Gold)
Rocket Man: Venus Star, Capt. Sand, Duckman Drake (Gold)
Outnumbered, But Never Outpunned!
Someone Has Got To Play...: Capt. Night (Gold)
Time to Split: Barby Gimp, Scourge Splitter (Gold)
Can't Handle This: Chasm, Hatchet Sal, Handyman (Gold)
Team Series B
Hack A Hacker: Krayola, Milkbaby (Gold)
Rice Cracker Rush: Riot Officer, The Master (Gold)
Superfly Lady: Capt. Pain (Gold)


Arcade League (Elite) bonuses:
One Shot Thrills
Babes in the Woods: Jo-Beth Casey (Gold)
Double Bill: Beetleman, The Impersonator (Gold)
Nikki Jinki Bricky: Nikki, Jinki (Gold)
Duel Meaning
If I'm Ugly-You Smell!: Mikey Twoguns, Jared Slim (Platinum)
Golem Guru: Fat Characters (Gold), Kypriss (Platinum)
Golden Thighs: High Priest, Aztec Warrior (Platinum)
Frantic Series
Hangar's Hat's Off!: Henchman, Dark Henchman (Gold)
Can't Please Everyone...: Maiden, Changeling (Gold)
Big Top Blowout: Mr. Giggles (Silver), Stumpy (Gold)
Team Series C
Bags Of Fun: Ringmistress, Big Hands, Slow Motion Deaths (Gold)
They're Not Pets!: Baby Drone, Bear (Gold)
Nice Threads: Small Heads, Compound level (Bronze), R-One-Oh-Seven (Gold)
Sincerest Form Of Flattery
Aztec The Dino Hunter: Dinosaur (Gold)
Half Death: Drone Splitter (Gold)
Dead Fraction: Jebediah Crump (Silver)


Challenge bonuses:
Glass Smash
Pane In The Neck: Rotating Heads (Gold)
Bricking It: Brick Weapon (Gold)
Stain Removal: Hunchback (Silver), Nothing (Gold)
Behead The Undead
Fight Off The Living Dead: Sewer Zombie (Gold)
Sergio's Last Stand: Sergio (Gold)
Day Of The Dammed: Feeder Zombie (Silver/Gold)
Infiltration
Silent, But Deadly: Viking Hat (Gold)
Trouble At The Docks: Pirate Hat (Gold)
Escape From NeoTokyo: Big Ears Hat (Gold)
Banana Chomp
Gone Bananas: Private Coal (Gold)
Monkey Business: Private Poorly (Silver), None (Gold)
Playing With Fire: Wood Golem (Silver)
Cut-Out Shoot-out
Take 'em Down: Ample Sally (Gold)
Fall Out: Marco the Snitch (Gold)
Pick Yer Piece: Sgt. Rock- Gold
Timesplitters "Story" Classic
Badass Buspass Impasse: Badass Cyborg (Gold)
But Where Did The Batteries Go?: R One-Oh-Seven (Platinum)
Hit me Baby One Morgue Time: Cropolite (Gold)
Monkeying Around
Simian Shooter: Insect Mutant (Silver)
Monkey Mayhem: Lola Varuska, Mischief (Platinum)
Dam Bursters: Robofish, Circus level (Gold)