Accessed from Level 1, Staging Area

Zombiemen: These are your weakest enemies, former marines firing small pistol bullets. They are first found in Level 1: Staging Area. Use any weapon to defeat them.

Shotgun Guys: Slightly stronger than Zombiemen, they wield shotguns that can deal significant damage on higher skill levels. They are also first found in Level 1: Staging Area. Any weapon can be used to defeat them.

Imp: These small, brown monsters attack with fireballs from a distance or by scratching up close. Their attacks are weak. Use any weapon except the Pistol or Fist (unless you have a Berserk) to defeat them in one hit. First found in Level 1: Staging Area.

Demon: These pink, bull-like creatures attack by biting, which can be very damaging on harder skill levels. Stay at a distance and use any weapon, or use a Chaingun or Chainsaw up close. First found in Level 1: Staging Area.

Spectre: Nearly invisible, Spectres are similar to Demons and attack by biting. Their attacks can be damaging on harder skill levels. Use any weapon, or a Chaingun or Chainsaw up close. First found in Level 2: The Terraformer.

Lost Soul: These flying, fiery enemies attack by charging. They often appear in groups and emerge from Pain Elementals. Use anything but a Pistol, Fist, or Chainsaw to defeat them quickly. First found in Level 4: Holding Area.

Cacodemon: These large, flying, red enemies shoot fireballs or bite up close. The Chaingun and Chainsaw are effective. First found in Level 3: Engineering.

Hell Knight: A significant threat, these tall, light-colored enemies attack with green fireballs or close-range scratches. Three rockets from a Rocket Launcher or three Shells from a Super Shotgun are effective, as are the Plasma Gun and BFG. First found in Level 4: Holding Area.

Baron of Hell: Darker than Hell Knights, these tougher enemies also attack with fireballs and scratches, but with more power. Use a Rocket Launcher, Plasma Gun, or BFG for a quicker fight. First found in Level 6: Alpha Quadrant.

Arachnotron: These spider-like enemies fire green cells. Almost any weapon except the Pistol, Fist, or Chainsaw can defeat them; three shots from a Rocket Launcher or Super Shotgun are effective. First found in Level 32: Hectic.

Pain Elemental: These large, flying, one-eyed enemies do not attack directly but spawn Lost Souls from their mouths. One BFG shot, or three rockets or Super Shotgun shells, will defeat them. Three Lost Souls will appear upon their death unless they explode near a wall. First found in Level 6: Alpha Quadrant.

Mancubus: Described as a fun enemy and a favorite. (No further details provided in source.)