They are discovered by finding a hidden lever in each level, that opens a secret door, and then entering the 16-bit texture mapped area hidden behind the opened door. This area consists of one section of the classic map that is unlocked, but with enemies from Doom (2016) replacing the classic enemies, and classic health, armor, and ammo pickups that do add to the player's Doom 2016 equipment.
Note that the enemies in the unlocked texture-mapped section are also considerably more difficult than they are in the main part of the map, because they fire more quickly and their projectiles are faster. This is possibly an unintended consequence of the simpler walls vastly reducing memory requirements. However, it is not necessary to kill them, as simply unlocking the secret area and entering it will both give the player credit for finding the secret, as well as unlock the Classic Map. Furthermore, many of the classic map fragments beginning with the Toxin Refinery fragment in the Argent Facility do not have any enemies at all, but do provide equipment for the player.
The Classic Maps are played in the same way as the small sections that unlock them do, with Classic map layouts, wall textures, and objects, but with the player's HUD, weapons, and enemies from the Doom reboot. However, to make things challenging, the player is provided only with the Doom 2016 Pistol rather than being allowed to bring in all his weaponry from the main game.
List of secret levels
Where they are found is in brackets.
- Entryway (The UAC)
- Hangar (Resource Operations)
- Nuclear Plant (Foundry)
- Toxin Refinery (Argent Facility)
- Halls of the Damned (Argent Energy Tower)
- Slough of Despair (Kadingir Sanctum)
- Underhalls (Argent Facility (Destroyed))
- Phobos Lab (Advanced Research Complex)
- Tower of Babel (Lazarus Labs)
- Phobos Anomaly (Titan's Realm)
- House of Pain (The Necropolis)
- Command Control (VEGA Central Processing)
- Pandemonium (Argent D'Nur)