TimeSplitters

Successfully complete the 1950 Village level in story mode on the easy difficulty setting to unlock the 1950 Village level in arcade mode; and "Period Horror" as a Bot Set in arcade mode.

Successfully complete all challenges to unlock the Challenge 3C Bowl Them Over level and the "All Enemies Are Robofish" cheat.

Successfully complete the first three levels in story mode on any difficulty setting to unlock the Village, Chemical Plant, and Planet-X levels in arcade mode. Additionally, Dr. Seth Graven, Dr. Katje Nadir, Fingers McKenzie, Ravelle Velvet, Sebastian Photon, and Angel Perez Bots will also be unlocked as playable characters in arcade mode.

Complete the following levels in each difficulty setting in the following time limit to open up extra options:

Tomb:
Easy - 1 minute - Paintball Mode
Norm - 3m 10s - Priestess Character
Hard - 6m - Teeth Mummy Character

Chinese:
Easy - 1m 15s - Chinese Waiter Character
Norm - 3m - Weird Gun Sounds Mode
Hard - 6m - Mr Big Character

Cyberden:
Easy - 50s - Siamese Cyborg Character
Norm - 1m 30s - Tuxedo Cyborg Character
Hard - 3m - Enemy Rockets Mode

Village:
Easy - 30s - All Heads Detachable Mode
Norm - 1m 30s - Hick Hyde Character
Hard - 4m - Insect Mutant Character

Chemical Plant:
Easy - 30s - Male/Female SWAT Characters
Norm - 1m 10s - Infinte Ammo Mode
Hard - 6m - Malehood Character

Planet X:
Easy - 55s - Green Alien Character
Norm - 1m 30s - Float Alien Character
Hard - 3m - No Heads Mode

Mansion:
Easy - 1m 45s - Big Head Mode
Norm - 5m - ?
Hard - 10m - Girl Zombie Character

Docks:
Easy - 1m 15s - Male/Female Soldier Charaters
Norm - 1m 30s - Big Hands Mode
Hard - 2m - Shock Trooper Character

Spaceways:
Easy - 1m 10s - Red Alien Character
Norm - 1m 20s - Female Alien Character
Hard - 2m - Small Heads Mode

If you do challenge 6-C Heist perfectly on your first try (meaning the opposition scores nothing), you will have an unbeatable record of 0... and never get the Veiled SWAT character on that profile, ever.