Is Doom A Megadungeon?

For those of you who don't know the rich backstory of the Doom franchise, let me catch you up on some of the lore;

• There are demons
• You have guns
• Bullets + Demons = Dead Demons
• Repeat

The main gameplay loop of doom is as follows;

• Arrive at a level
• Try to find a way to the next level
• Find keys, avoid traps, and kill monsters to successfully reach the next level to continue the game

Does this sound familiar? It seems awfully close to a standard dungeon crawl, minus the treasure of course. "Treasure" in Doom is mostly ammunition, health kits, armor, and power-ups. In addition, there is no gaining levels, and you can respawn at the start of a level when you die. But at its core, Doom has the makings of a megadungeon game.

Lets take a look at one of the maps for Doom

This is The Hangar, the first level of the game. Its a little hard to read what is there, just from the map to the untrained eye, but it contains 2 traps (pools of acid), 3 secret doors, and some combat encounters, along with "treasure" in the form of ammo, health, a shotgun, and armor.

If I wanted to, I could modify this to fit a fantasy setting and run it as the entrance to a demonic temple. The monsters in this level are possessed soldiers (zombiemen), and spiky demons that shoot fireballs (imps). If I just change the soldiers' equipment to be leather armor and swords instead of rifles and a combat vest, suddenly they could easily be zombie guards animated by the demon cult to protect their secrets. The shotgun could just be a magic item of some sort, such as a wand of magic missile.

And now, here is a list of reasons why Doom might not be a megadungeon:

• No role-playing whatsoever, with no NPCs (this changes in Doom 3 and Doom 2016 to a certain extent, as well as the two Doom RPGs for mobile devices)
• The player character grows more powerful through acquiring items, not upgrades to health and attacking.
• The player character cannot leave the "dungeon", rest in town, and come back later. 
• You can't pay hirelings to accompany you to act as meat shields.

To finish off this mess of a post, here are some demons from Doom (and Doom 2) converted to my house rules (Images taken from doomwiki.org):

Imp 

HD 1

AC light

ATK 1 claw or 1 fireball

DMG 1d6+1 or 1d6

HDE 1

Imps are brown, furry humanoids with bony spikes protruding from their bodies. They may shoot small fireballs as projectiles, but prefer to tear their victims apart with their claws.

Pinky/Spectres

HD 3

AC medium/heavy with shield

ATK 1 bite

DMG 1d6+2

HDE 3/4

Named for their skin color, pinkies are violent, gorilla-like demons with a maw of sharp teeth. Some pinkies have evolved to be transparent, resulting in a higher armor class and the ability to hide in darkness without being noticed 5 out of 6 times. These nearly-invisible pinkies are called spectres.

Lost Soul

HD 1

AC medium

ATK 1 bite

DMG 1d6

HDE 1

These lesser demons are large, flying skulls with horns that are on fire. They are not particularly intelligent, and some believe this is what happens to the souls of sinners after the demons torture them over the centuries. 

Pain Elemental

HD 5

AC heavy

ATK none

DMG none

HDE 10

Special: Each round, a Pain Elemental may vomit up one Lost Soul, which may make its action immediately after the Pain Elemental. If killed, a Pain Elemental explodes into a group of 1d6 Lost Souls.

These demons take the form of one eyed, brown orbs with tiny vestigial arms. Their mouth can unhinge, allowing them to disgorge Lost Souls at will. It is thought that Pain Elementals act as living portals to Hell.

Archvile

HD 7

AC heavy

ATK 1 flaming gaze (automatically hits as long as the target is within line of sight)

DMG 1d6+2

HDE 10

Special: Each round, an Archvile can choose to restore a dead demon within melee range of it back to life (with full HP restored) instead of attacking.

The archviles are foul demonic necromancers, capable of bringing their fallen allies back from the dead. Their very gaze can set their victims on fire.