This is a weird secret, but to access Hectic (the final level), you must go to Level 1 (Staging Area). At the beginning you will see an exploding barrel. Don't shoot it. There are 10 of these barrels in this level. Shoot every barrel except for the one at the very start. Now, when you have done so, use the teleporter through the Blue Door, to get back to the very start.
Look behind you and you will see a door. Now go over to the last remaining barrel and shoot it. Now run through the door and look beside the blood stain that is on the ground. A teleporter will be there, be quick to get to the teleporter because it doesn't stay there for long.
Access Level 29: Outpost Omega Without Passwords
This level can be accessed through the Holding Area. Near the end of the level, you will see 4 switches. Flick them in the following order: 3, 1, 2 and 4. The circular platforms ahead should now line up. Now go through the small door and get into the teleporter.
Now you will appear in line with the 4 circular platforms. Run across them and go into the red room. Kill the Nightmare Imp and exit the stage. This is a Secret Exit which will have you go straight to Outpost Omega. Be warned that you will only have the guns that you exited the Holding Area with.
Access Level 30: The Lair Without Passwords
This level can be accessed through the Altar Of Pain level. To get there, after grabbing the Yellow Key, simply jump to the platform with the Soulsphere on it. Now do not jump off this platform. Look at the map and you will notice that the platform you are standing on looks like an arrow. Turn in the direction that the arrow is facing. Now run into the wall exactly where the arrow is pointing. You will go straight through the wall.
Now be careful as a Baron Of Hell roams this area. Kill it and exit the stage. You will now be sent to The Lair level.
Access Level 31: In The Void Without Passwords
This level can be accessed through the Spawned Fear level. To get there, after going through the Yellow Door and walking up the steps, you will hear a door open. Quickly run back through the Yellow Door and turn left and then turn left again and enter the room there (the one where you faced 3 Hell Knights). You will see a small area with a teleporter exit in it. Run for it before it disappears and that is it.
You will now be taken straight to Level 31 In The Void. If you fail to make it to the teleporter, just repeat the steps above, there is no need to restart the level.
All passwords from all levels
The Terraformer - CB92 NBPL SYL? JO27
Main Engineering - BXYH ?G41 6Z4J PJ?Z
Holding Area - CYCC MGPK X47G TS2B
Tech Center - CF3? PG6D S12Z PFKB
Alpha Quadrant - BXRO TH1F 52GG 7W?B
Research Lab - BBXW HLGS XB8F 4RKB
Final Outpost - FVV9 FL55 QGFV DWJB
Even Simpler - FFLB MQ6C VV1C PF1B
Cyberdemon Survival
There are simple methods to defeating the Cyberdemon. Yes it is very strong but its missiles are actually quite slow. When it launches missiles, unless you are near a wall, where the impact of the missile is still felt, you can simply dodge the missiles with simple right to left movements. This is probably the easiest in the Watch Your Step and No Way Out levels. Soon you will be confident enough to take on Cyberdemons with nothing but a Pistol!
Doom Artefact 3
In the In The Void level, get right to the very end and slowly walk over to the edge of the platform (the platform whereby if you jump off the edge the stage ends) and when the screen shakes, go to where you turned on the Blue Key Switch. The pillar that had the Doom Artefact will now have a switch on it. Use the switch and run all the way back to the Red Key door (quickly) and run towards the wooden bridge.
The artefact will be at the end of the bridge. Grab the artefact and then up the top should read: 'It must do something'. Now you can exit the stage.
If the artefact disappears before you can snare it, just repeat the entire method, right from the start.
Doom Artifact 1
In the Outpost Omega level, once you have killed everything and reached the exit (this makes it easier), turn back. Go to where you entered the level and walk to the 4 computers that are just before the room with the 4 cages. Turn Right and activate the computer that isn't flashing. The Red Key will be visible at the very end of the room of cages. Now go and get it (if you haven't already).
Now return to the Blue Key Door. Jump down to the Platform below. Now turn left and jump down to the platform below. Now look to your right and you will see another platform. Run across to it and look at the wall on the left. It is actually a lift. Bring it down and shoot the switch that you will see directly across from it. A Laser Gun should appear in front of you. Take it and turn right. Kill the Imp and run across to the platform that his body is on.
The switch there temporarily lowers a lift that is 3 platforms away (behind you). Flick the switch and quickly run across to the lift and get on it. When ir rises again, look across and you will see a switch on the wall. Shoot it and jump off the lift.
Now return to the platform that the dead Imp is on and flick the switch again and run for the lift that you were on before. The switch that you shot just before, will be there again. Shoot it and jump from the lift again. Now turn left and run to the 'next' platform (not the one with the dead Imp). Turn right and jump down to the platform below. Now to your right again you will see a door. Use the Red Key to open it and grab the artifact.
Note: There are 3 of these artifacts. Each of them making the Laser more powerful.
Doom Artifact 2
In The Lair level, get to the room where you see a Doom Artifact at the top of a huge pillar. To get the artifact, flick the skull switch behind you. 3 sets of stairs will appear. Walk up the middle set. Now turn left and you will see an area with a wooden floor. When you step on this floor, a switch will appear high on a wall near the pillar. Walk over to the window that overlooks the area and look around. If you don't see the switch you can try again.
Once you see it, shoot it and look in the small square window that slightly overlooks the stairs. Look through it and you will see another square window and another switch will be visible through it. Shoot the switch.
Now the Doom Artifact will be at ground level and will be ready for the taking.
Playground Advice
When you are on the Playground Level, the key to dodging the bullets from Arachnitrons, is simply to not stop moving. Simply keep running and you will not be hit. As for the Cyberdemon that emerges after you defeat the Arachnitrons, simply keep your distance from it and fire at it. When it returns fire, simply run behind one of the four large pillars and you should be safe. As for the Mancubis' that com afterwards, they will be easy to kill. Pick them off one at a time. By the time that only two remain, you will be fine.
It is, however, important to try to not grab the Partial Invisibility items as they make enemy fire unpredictable and hence, harder to dodge. After defeating the Arachnitrons, you should grab every box of Rockets and wait until you become visible again before you grab the Megasphere.
Super Password
Enter W93M 7H2O BCYO PSVB. This code will give you 100 health, 200 armour, all weapons, full ammo and a backpack. It'll also supply you with the three pentagram icons that make your laser three times stronger than the BFG and can kill bosses in four seconds flat. These useful pentagrams also let you use the three mysterious switches in the last level, closing the gates which spawn the monsters.
Very cool DOOM cheat
In Doom 64, enter this password ?TJL-BDFW-BFGV-JVVB. At any time during a level, pause it and there will be a new option called FEATURES. IT will give things like invincibility, all weapons, etc. In the first feature choose the level you want and press a C button to go to it.