Enter PROHIAS as a code.
Enter ANTONIO as a code.
Enter MADMAG as a code.
Enter DISGUISE as a code.
Successfully complete the Mansion level in story mode.
Successfully complete the Kooky Carnival level in story mode.
Successfully complete the Space Ship level in story mode.
Successfully complete the Volcano Lair level in story mode.
Successfully complete the Space Ship level in story mode.
Successfully complete Area 51 level in story mode.
Successfully complete the Robot Factory level in story mode.
Successfully complete the Oil Rig level in story mode.
Successfully complete the Haunted House level in story mode.

From the main menu, go to the "Extras" screen. Then go to "Cheats" and there's a field to enter Passwords. Enter "FAIRY" and press "Enter."

From the main menu, go to the "Extras" screen. Then go to "Cheats" and there's a field to enter Passwords. Enter "ARMOR" and press "Enter."

Beat every level in Single Player Modern Mode to unlock the Timed Levels. For each Timed Level you beat, you unlock the concept art for that level.