Level Codes:
On the Title Screen, click on the Options and from the Options, choose Password and then enter these codes:

CQXK QBXT SY1? DSK7 Level 2 - The Terraformer
B9MG 2H9P 4KNH ZZ2Z Level 3 - Main Engineering
C?LB 6MN2 WTGF WJKB Level 4 - Holding Area
CSP2 6NWZ ?8PF 6TKB Level 5 - Tech Center
B96Z HSWT 3CTD FC?B Level 6 - Alpha Quadrant
B5JD 4WB6 048W 2DKB Level 7 - Research Lab
F90C H111 XC4T XV9B Level 8 - Final Outpost
FSSX H1WB ZDF? 6B1B Level 9 - Even Simpler
D7F7 HDZT LFTD FR9B Level 10 - The Bleeding
NQ1K 691F ??CR PGGB Level 11 - Terror Core
M99G HJY1 3FDN YRQB Level 12 - Altar of Pain
4SNQ W3GW PPXY T9VB Level 13 - Dark Citadel
383S WGCS RC2P TLLB Level 14 - Eye of the Storm
3RNL 4CC5 CVYP ?GLB Level 15 - Dark Entries
29HC 4HH1 Y1TR PV3B Level 16 - Blood Keep
2QK4 6HMQ F7GR 42BB Level 17 - Watch Your Step
1748 HSWL NC9? F4BB Level 18 - Spawned Fear
1QFP Mb2N KZHM F73B Level 19 - The Spiral
07VL HDX1 BCQR YDLB Level 20 - Breakdown
ORT? 4RD7 T4P9 K?BB Level 21 - Pitfalls
Z9MG ?W79 64X4 873B Level 22 - Burnt Offerings
ZR67 ?H8G M1?Q ZPBB Level 23 - Unholy Temple
Y7Z3 071G CT74 JGVB Level 24 - No Escape
?TJL BDFW BFGV JVVB Level 25 - Cat And Mouse (Under "Features")
?TJL BDFW BFGV JVVB Level 26 - Hardcore (Under "Features)
?TJL BDFW BFGV JVVB Level 27 - Playground (Under "Features")
W7JM OLX1 FRKR KMLB Level 28 - The Absolution
RQ81 HNX6 7FOH 5??B Level 29 - Outpost Omega
V8C6 W3HL KPP4 1HBB Level 30 - The Lair
VRL5 6WQV H?WP LTVB Level 31 - Into the Void

Play Hectic Demo:
At the password prompt, enter this password. Then, return to the title screen and wait for the game's introduction screen to show (just wait for about 30 seconds, and the screen will change). Now wait for the 3 demos. After the 3rd demo, there will be a 4th demo, showing how to beat Level 32: Hectic.

RVNH 3CT1 CD3M 0??? In the 3-demo cylce, a fourth demo will appear showing how to beat Level 32: Hectic

Enable Features menu and start new game with Everything:
Press Start to pause the game, select Options to bring up the Options menu, select Password, and enter one of these passwords. Each password enables the Features menu (cheat-options menu), and gives the player 200% health, 200% armor, a backpack, maximum weapons and ammo, and all 3 Demon Artifacts.

?QDM 7HYC BB4X FJVB - Start with Everything at "BE GENTLE!" Skill
?LD4 7HYB BB4D CJVB - Start with Everything at "BRING IT ON!" Skill
?GFM 7HYT BB9X KHVB - Start with Everything at "I OWN DOOM!" Skill
?BF4 7HYS BB9D HHVB - Start with Everything at "WATCH ME DIE!" Skill

Skip some levels on Skill Level:
Be Gentle Go to the Password screen and enter the following:

cdp8 9bj2 68zt svk? - Skip Level 02 on Skill Level Be Gentle
cxm8 9bjy 681t jvk? - Skip Level 03 on Skill Level Be Gentle
ddk8 9bjt 683s 9vk? - Skip Level 04 on Skill Level Be Gentle
dxh8 9bjp 685s 1vk? - Skip Level 05 on Skill Level Be Gentle
fdf8 9bjk 687s svk? - Skip Level 06 on Skill Level Be Gentle
fxc8 9bjf 689s jvk? - Skip Level 07 on Skill Level Be Gentle
gd?8 9bc? 69br ?bk? - Skip Level 08 on Skill Level Be Gentle
gx88 9bc6 69dr 2bk? - Skip Level 09 on Skill Level Be Gentle
hd68 9bc2 69gr tbk? - Skip Level 10 on Skill Level Be Gentle
hx48 9bcy 69jr kbk? - Skip Level 11 on Skill Level Be Gentle
jd28 9bct 69lq ?bk? - Skip Level 12 on Skill Level Be Gentle
jx08 9bcp 69nq 2bk? - Skip Level 13 on Skill Level Be Gentle
kdy8 9bck 69qq tbk? - Skip Level 14 on Skill Level Be Gentle
kxw8 9bcf 69sq kbk? - Skip Level 15 on Skill Level Be Gentle
lft8 9bb? 69vp ?vk? - Skip Level 16 on Skill Level Be Gentle
lyr8 9bb6 69xp 2vk? - Skip Level 17 on Skill Level Be Gentle
mfp8 9bb2 69zp tvk? - Skip Level 18 on Skill Level Be Gentle
mym8 9bby 691p kvk? - Skip Level 19 on Skill Level Be Gentle
nfk8 9bbt 693n ?vk? - Skip Level 20 on Skill Level Be Gentle
nyh8 9bbp 695n 2vk? - Skip Level 21 on Skill Level Be Gentle
pff8 9bbk 697n tvk? - Skip Level 22 on Skill Level Be Gentle
pyc89bbf699n kvk? - Skip Level 23 on Skill Level Be Gentle
qf?8 9bf?6?bm ?bk? - Skip Level 24 on Skill Level Be Gentle
qy88 9bf66?dm 2bk? - Skip Level 25 on Skill Level Be Gentle
rf68 9bf2 6?gm tbk? - Skip Level 26 on Skill Level Be Gentle
ry48 9bfy 6?jm kbk? - Skip Level 27 on Skill Level Be Gentle
sf28 9bft 6?ll ?bk? - Skip Level 28 on Skill Level Be Gentle
sy08 9bfp 6?nl 2bk? - Skip Level 29 on Skill Level Be Gentle
tfy8 9bfk 6?ql tbk? - Skip Level 30 on Skill Level Be Gentle
tyw8 9bff 6?sl kbk? - Skip Level 31 on Skill Level Be Gentle
vbt8 9bd? 6?vk 9vk? - Skip Level 32 on Skill Level Be Gentle

Skip to some levels in Skill level:
Bring it on Go to the Password screen and enter the following codes:

cjpr 9bj1 68z? qvk? - Skip to level 02 on skill level: Bring it on
c1mr 9bjx 681? gvk? - Skip to level 03 on skill level: Bring it on
djkr 9bjs 6839 7vk? - Skip to level 04 on skill level: Bring it on
d1hr 9bjn 6859 zvk? - Skip to level 05 on skill level: Bring it on
fjfr 9bjj 6879 qvk? - Skip to level 06 on skill level: Bring it on
f1cr 9bjd 6899 gvk? - Skip to level 07 on skill level: Bring it on
gj?r 9bc9 69b8 8bk? - Skip to level 08 on skill level: Bring it on
g18r 9bc5 69d8 0bk? - Skip to level 09 on skill level: Bring it on
hj6r 9bc1 69g8 rbk? - Skip to level 10 on skill level: Bring it on
h14r 9bcx 69j8 hbk? - Skip to level 11 on skill level: Bring it on
jj2r 9bcs 6917 8bk? - Skip to level 12 on skill level: Bring it on
j10r 9bcn 69n7 0bk? - Skip to level 13 on skill level: Bring it on
kjyr 9bcj 69q7 rbk? - Skip to level 14 on skill level: Bring it on
k1wr 9bcd 69s7 hbk? - Skip to level 15 on skill level: Bring it on
1ktr 9bb9 69v6 8vk? - Skip to level 16 on skill level: Bring it on
12rr 9bb5 69x6 0vk? - Skip to level 17 on skill level: Bring it on
mkpr 9bb1 69z6 rvk? - Skip to level 18 on skill level: Bring it on
m2mr 9bbx 6916 hvk? - Skip to level 19 on skill level: Bring it on
nkkr 9bbs 6935 8vk? - Skip to level 20 on skill level: Bring it on
n2hr 9bbn 6955 0vk? - Skip to level 21 on skill level: Bring it on
pkfr 9bbj 6975 rvk? - Skip to level 22 on skill level: Bring it on
p2cr 9bbd6995 hvk? - Skip to level 23 on skill level: Bring it on
qk?r 9bf9 6?b4 8bk? - Skip to level 24 on skill level: Bring it on
q28r 9bf5 6?d4 0bk? - Skip to level 25 on skill level: Bring it on
rk6r 9bf1 6?g4 rbk? - Skip to level 26 on skill level: Bring it on
r24r 9bfx 6?j4 hbk? - Skip to level 27 on skill level: Bring it on
sk2r 9bfs6?13 8bk? - Skip to level 28 on skill level: Bring it on
s20r 9bfn 6?n3 obk? - Skip to level 29 on skill level: Bring it on
tkyr 9bfj 6?q3 rbk? - Skip to level 30 on skill level: Bring it on
t2wr 9bfd 6?s3 hbk? - Skip to level 31 on skill level: Bring it on
vgtr 9bd9 6?v2 7vk? - Skip to level 32 on skill level: Bring it on

Skip levels on I own doom:
difficulty Enter these at the password screen to skip to that level.

CNN8 9BJ0 680T NVK? - Level 02
C5L8 9BJW 682T DVK? - Level 03
DNJ8 9BJR 684S 5VK? - Level 04
D5G8 9BJM 686S XVK? - Level 05
FND8 9BJH 688S NVK? - Level 06
F5B8 9BJC 68?S DVK? - Level 07
GN98 9BC8 69CR 6BK? - Level 08
G578 9BC4 69FR YBK? - Level 09
HN58 9BC0 69HR PBK? - Level 10
H538 9BCW 69KR FBK? - Level 11
JN18 9BCR 69MQ 6BK? - Level 12
J5Z8 9BCM 69PQ YBK? - Level 13
KNX8 9BCH 69RQ PBK? - Level 14
K5V8 9BCC 69TQ FBK? - Level 15
LPS8 9BB8 69WP 6VK? - Level 16
L6Q8 9BB4 69YP YVK? - Level 17
MPN8 9BB0 690P PVK? - Level 18
M6L8 9BBW 692P FVK? - Level 19
NPJ8 9BBR 694N 6VK? - Level 20
N6G8 9BBM 696N YVK? - Level 21
PPD8 9BBH 698N PVK? - Level 22

Features Menu (the alternate way):
On any skill level, beat Level 32: Hectic using the exit where you need each key. Then, for the rest of the game, there will be a Features menu when you press Pause (it will disappear if you shut off your Nintendo 64 or you exit the game).

Hectic - Map 32:
To unlock Hectic, you need to go to the first level, and shoot EVERY explosive barrel in the level, EXCEPT for the first one you see when on the staging platform. After you shoot the other 9 barrels, use the teleporter (Hidden in one of the walls in the main corridor) to return to the start of the level. Then, shoot the barrel you left and turn directly behind you, and go into the room with the berzerk pack. There will be an open teleporter- a blood stain marks the entrance. Be sure to hurry- it doesn't stay open long!

Once you get here, good luck- some consider this one of the hardest Doom levels ever made! Once you beat this level, you unlock the Features menu.

Super Duper Speed:
You can exploit the N64 controller's "Neutral position reset"; feature easily in this game. What you do is hold down on the control stick, then simultaneously press L+R+Start. What this does is set the "down" position on the joystick to neutral. So what happens when you press up? You go REALLY FAST! Problem with this is that the neutral position is now "Down." To fix it, just let the joystick go neutral, then press the button combination again.

This works GREAT in the first hidden level, "Outpost Omega" when trying to do the switch puzzle to get the artifact.