To get the password cheat in the beginning of the game go in to the option labeled "password" then enter "?TJL BDFW BFGVJVVB". Then when you play the game you will get an extra option which reads password.

To do these cheats you must play the game on skill level "watch me die" level passwords:
level 2: csnr 9bjz 680? lvk?
level 3: c9lr 9bjv 682? bvk?
level 4: dsjr 9bjq 6849 3vk?
level 5: d9gr 9bjl 6869 vvk?
level 6: fsdr 9bjg 6889 lvk?
level 7: f9br 9bjb 68?9 bvk?
level 8: gs9r 9bc7 69c8 4bk?
level 9: g97r 9bc3 69f8 wbk?
level 10: hs5r 9bcz 96h8 mbk?

For 100 health, 200 armor, a backpack, levels 29 & 31 finished, start on the last level and get 3 pentagram items, just enter the password as w93m7h2abcyopsvb .

Final level passwords:
skill 1: vbt89vd?6?vk9vk?
skill 2: vgtr9bd96?v27vk?
skill 3: vls89bd86?wk5vk?
skill 4: vqsa9bd76?w23vk?

The Features Menu - There are no Key Sequence Cheat Codes in Doom 64. However, there IS in fact a cheat menu in the game. In order to reach the menu, you do NOT enter a key sequence like in most games, but rather, you must first discover the hidden Level 32: Hectic, then be able to survive it and collect all three keys in it before leaving. Once done, you will then have access to the "Features" cheat menu.

Finding Level 32: Hectic  -  To reach Level 32: Hectic, you must start a new game. Go through Level 1: The Staging Area and destroy ALL the barrels in the level except the one to the right of start. There are a total of 10. Remember the secret areas, the room where the exit is, and the alcove in the hallway leading to the exit. Be sure to collect the Soul Sphere and Combat Armor for a better chance at surviving Level 32.

Once you've destroyed all of the level's barrels, take the teleporter that you can access from the secret area behind the Blue Door. When you reappear, destroy the barrel ahead of you on your right and enter the door behind you. A secret area will temporarily open on the left wall that leads to Level 32: Hectic.

Walkthru: Obtaining the Three Keys in Level 32: Hectic
At the beginning of Level 32: Hectic, enter the door to the right for the Red Key. Once you pick it up, you'll be dropped into a room with three Arachnotrons. Quickly go left or right to get the Rocket Launcher. Destroy the Arachnotrons and return to the door that leads to this room. Beware of the crushers that come down from the ceiling, search the wall to lower it and let it raise you back to the main room.

Enter the door on the other side of the room to get the Blue Key. As soon as you get it, run back to the door you entered from. The ceiling will crush you if you stay in the alcove where you find the Blue Key. Four Hell Knights will appear and the door you came in from will be closed. Destroy them with your Rocket Launcher to be able to leave the room.

Enter the door on the left side of the room, at the opposite wall is the Yellow Key. The floor rises in different parts of the room, and darts will fire from the faces in there. Try your best to avoid damage while getting the Yellow Key.

Exit the room to return to the start. Directly across from you is a switch you will now be able to access. Having the keys, you can now remove the bars and press the switch to exit Level 32: Hectic and gain access to the "Features" cheat menu. Pause the game and "Features" will be the fourth selection down.

Options in the Features Menu
Warp to Level: This is the ONLY way you can access Level 25: Cat and Mouse, Level 26: Hardcore, and Level 27: Playground. Three secret "fun" levels in between Level 24 and the final level, 28: The Absolution.
Invulnerability: Turn "God Mode" ON/OFF using Button A or B. This is equal to using IDDQD in the original PC Doom.
Health: Use Button A or B to restore your health.
Weapons: Use Button A or B to get all weapons and ammo for each. This is like using IDKFA in the original Doom PC.
Map Everything: Turn Map Everything ON/OFF using Button A or B. You will have a full map of the level.

The Ultimate Password  -  It takes skill to cheat in Doom 64, but... For those of you who are either too lazy to try level 32 or tried it and just plain and simply can't get through it, here's the password to obtain access to the "Features" cheat menu as well as all weapons and full ammo for each. ?TJL BDFW BFGV JVVB It takes you to Level 1: The Staging Area with all weapons and ammo, just pause and the fourth selection down is "Features". Thanks goes out to Dave Senger for this password.

Super Password  -  Here's a little password for those of you who like to take shortcuts for better chance of survival. W93M 7H2O BCYO PSVB It takes you to Level 28: The Absolution, and gives you 100% Health, 200% Armor, all weapons, full ammo, backpack, and all three Special Keys (from Levels 29, 30, and 31) which pumps up the Laser Rifle and allow you to seal off the gates from which monsters enter the area where you'll be fighting Mother Demon.

Level 1: "The Staging Area" ?TJL BDFW BFGV JVVB
Level 2: "The Terraformer" CL7K LBPT S2PT Q0T8
Level 3: "Main Engineering" B6MB 8G3P WV0J M2?0
Level 4: "Holding Area" F3CR TLPB N0GH G6KW
Level 5: "Tech Center" FPVD TLPV Z0KH RK2W
Level 6: "Alpha Quadrant" B3L7 8M7? M6DY 4GKC
Level 7: "Research Lab" BNJ0 4SVX 64CD PBTC
Level 8: "Final Outpost" F394 K1X1 DKX? X0SC
Level 9: "Even Simpler" FPSZ F1DD 3KH? KLSC
Level 10: "The Bleeding" D3PN K50B GGBQ FDJC
Level 11: "Terror Core" NTLG HXWD 3FVS PFQB
Level 12: "Altar of Pain" M?HC HSVJ XDML YQ7B
Level 13: "Dark Citadel" 4QZM F1P5 DBG9 4PLB
Level 14: "Eye of the Storm" 3?2H ?108 62GS FPBB
Level 15: "Dark Entries" 3T5F CSH5 1D4L 2QLB
Level 16: "Blood Keep" 29D0 ?1R1 66MM WYBB
Level 17: "Watch Your Step" 2RWN 61RJ HYM4 3S3B
Level 18: "Spawned Fear" 19BL BDHR BBNZ ?ZVB
Level 19: "The Spiral" 1SJG HX0N 3FNL F1LB
Level 20: "Breakdown" 09VF HS11 1FZS YDLB
Level 21: "Pitfalls" 0SY1 4515 8576 6BVB
Level 22: "Burnt Offerings" Z8L3 DDL8 BC00 CQ3B
Level 23: "Unholy Temple" GGRS P3Q7 Q0C6 89QB
Level 24: "No Escape" Y1ZL VBGY CL11 FWBB
Level 28: "The Absolution" WO4R 6HRZ P9R9 ZSLB