The Doom series has always been home to a ton of secret areas, but the N64 exclusive Doom 64 took things to another level. Utilizing a different engine from what id Software created, the developers at Midway were able to pack denser geometry and better-scripted events into the game. This led to secret levels and items that had pre-requisites for finding. No simply running to a hidden switch and exiting.
Apart from experiencing more Doom 64, the secret levels in this overlooked classic actually house items that will help you on your journey: the Demon Keys. Upon collecting all three Demon Keys, players will receive an upgraded Unmaker weapon that puts the BFG 9000 to shame. It'll also make the last boss a complete breeze, even on the hardest difficulty.
So if you're looking to kick some ass and find every hidden level in Doom 64, this guide will get you there with little guesswork.
Each video is set to the specific time code for the appropriate level.
Map01: Staging Area
The very first hidden level can be accessed from the start of the game. In a mystery that probably took people ages to discover, you'll need to shoot every explosive barrel on the first map, then run to a teleporter at the start of the area. The first barrel you'll find will be at the very beginning, but you'll want to save that for last to give yourself more time.
The locations are rather hard to describe, but this video will help you spot them. There are 10 barrels in total and shooting them all will send you to Map32: Hectic. If this sounds like too much work, you can safely skip this secret and not miss any of the Demon Keys. Hectic mainly gives you earlier access to the rocket launcher, not to mention it unlocks the "Features" menu on the N64 original. It also has a bunch of items guarded by traps that will kill you if you go for them.
Map04: Holding Area
The next secret level is found on the fourth map in Doom 64. Towards the end of the level, you'll see some circular columns lined up towards a door. At an elevated platform overlooking these circular columns, you'll need to activate the switches in a specific pattern to create a bridge you can sprint over. Looking straight at them (going left to right), you'll need to press them in this order: 3, 1, 2, 4.
With that done, use the teleporter to the right of the exit and sprint across the columns. You'll then enter a secret room that contains the exit to Map29: Outpost Omega. This will have the first Demon Key.
Map12: Altar of Pain
The third secret level can be found on the twelfth map of the game. Progress as normal until you collect the yellow key, then survey the area. You'll spot a switch near the lower level and a pillar with a Soulsphere on it. You'll have to make a running jump from where the key was to the platform with the sphere. You can then check your automap to see an arrow pointing towards a hidden wall. Run through that, and you'll be in the secret exit room. Kill the enemy and proceed to Map30: The Lair.
Map18: Spawned Fear
The final hidden level is accessed rather late in the game. On the eighteenth level, after opening the yellow door (the final locked door in the level), you'll travel down a small set of stairs, then up another one while working towards the regular exit. As you're doing this, a door will open somewhere behind you. You'll need to turn around immediately, run through the yellow door, take a left, another left, and run into a small cave. You'll spot a teleporter that recently opened, sending you to Map31: The Void.