Unlock Mental Mode
Finish the game one time using Serious Mode. The mental Mode makes enemies phase in and out.
At the menu, click down the Left-Analog stick and press Black, White, Y. The Cheats option will appear in the menu.
Enter: A, B, B, A, Y, Y, X.
Enable the "Cheat mode" code. Go to the Alley Of The Sphinxes level. When there, instead of going straight into the level, turn around and head for the mountains. After a while you will begin to lose health very fast. Give yourself full health when you are in danger of dying. In a short while, you will get to the top of the mountain and stop losing health. You may want to get full health because you will lose about 30 when you fall to the other side. You will now be in more desert. Keep running for about twenty minutes and you will walk off the desert and enter an area that looks like all sky. While here, when your gun moves it looks strange. When you slice your knife around, the whole screen will get covered by an image of it. The best part about this area is that you can fly. Hold Jump to do so. You can travel anywhere in the level. To land, hold Crouch.
When you play in one player mode, you will get Serious Sam. After the first level loads and saves, exit. Go to one player mode and start a new game by pressing Y. A messed up name will appear. Do not delete that name; just select "Done". Allow the game to load and save, then exit and go to one player mode. If done correctly, Serious Sammy should be there. Keep doing these steps until you get all the charcaters. Note: When you get Serious Sam 2, do not select "OK"; there is no difference.
View programmers #3
On the "The Bridge" level, after awhile into the level you will find a trail of +5 health pills. At the end of the trail there is a Kleer Skeleton and a small building. Go inside and get the Gold Cross. Next to the building there are five graves. Press Use on them. They will open to reveal five big-headed people that resemble the programmers. They are the same as the ones in the first level except that they have horns. When you leave them alone, they will walk around like zombies with their arms out saying, "Come to daddy!" or something similar. When you walk up to them they will say, "Hey, wait!" and wave at you.
View programmers #2
There is a second group of programmers in the first level. From the start of the level, go down the first ramp, make a left turn, and look for a door that will not open. If you can find a secret rocket launcher on this mission, use it to destroy the door and release the second group of fans.
View programmers #1
In the first level, turn around and head down the ramp. Then, go right and turn towards the direction you are supposed to go. You should see a door. When you enter, you will encounter many big headed people that represent the game's developers. Also, if you go down the path that leads to the desert, many big red Biomechs will appear.
In the Great Pyramid stage, just before you enter the pyramid, you may have noticed a panel above the entry with alternating I's and O's. This is a timer counting down in binary. The timer starts at 1,023 and goes down from there.
Destroy all the statues in the Valley Of The Kings level.
Click and hold the Left Analog-stick and press Black, White, Y(3) at the main menu. This will unlock the "Cheats" and "Developer Cheats" options. The "Developer Cheats" will allow you to go to certain checkpoints.
Unlock Mental Mode:
Finish the game with Serious Mode.
In the main menu click and hold the left analog-stick, while holding it press BLACK, WHITE, then Y. If done right a cheat menu will appear at the main menu.