The game has many weapons taken from Doom and Doom II, but it also took some weapons from other Build engine games. It also featured Doomguy talking. The enemies are from Doom and Doom II, too. However, the music isn't from the Doom games (only the title screen uses Doom music, and most of the music in the levels is from Blood).
The gameplay differs depending on whether or not the doom.con is activated. With the .con activated, the following changes occur:
The player starts off with 110% health by default instead of 100%
Uzi can hold 400 bullets maximum
Double-barreled shotgun can hold 20 shells maximum
Chaingun Cannon can hold 400 bullets maximum
Some of the items are named differently
The Plasma Cannon functions similarly to the Plasma Gun/Plasma Rifle from Doom in that it doesn't freeze enemies
The Hell Knight "statues" (they just stand still and are incapable of being killed), along with the Doom health bonus and armor bonus sprites (which are not able to be picked up, thus rendering them useless), are removed
Some of the enemies and bosses have Doom enemy voices (the Cacodemon sounds like it does in Doom rather than using Octabrain sounds) instead of Duke Nukem 3D enemy voices
The chaingun cannon ammo boxes give the player 100 bullets instead of 50
The Doomguy walks at a slower speed
The mod doesn't use Duke Nukem 3D music and instead uses music mostly from Blood
Revenants use projectiles that function like rockets (although the projectile looks like a Mancubus fireball) instead of shrinking the player
Without the .con activated, the previously listed changes are not present (Revenants will shrink the player, enemies and bosses will use Duke Nukem 3D sound effects and voices, etc).
The forces of Hell have invaded Earth, and the Doomguy is trying to stop them. The Doomguy battled his way through demons and zombies, reaching a portal where he confronted the Spiderdemon. The Spiderdemon is killed, and the Doomguy teleports to a city where the Mancubus is hiding. The town is secured, and the Mancubus is killed. The Doomguy teleports to Hell, where he defeated both the Arch-vile and the Cyberdemon, thus ending the invasion.
In Duke's Realm, it is impossible to normally complete the level due to the red access chip being behind a door that the player cannot open. This means that the player has to use the no-clip cheat to go through the door or use a cheat to get the red access chip.