According to the game's executive producer Marty Stratton, the key principles of Doom's single-player mode are "badass demons, big effing guns, and moving really fast".[1] The game allows players to perform movements such as double jumps and ledge climbs throughout levels of industrial and corporate fields of a Union Aerospace Corporation (UAC) research facility on Mars and then levels of Hell,[2] as the combat system puts emphasis upon momentum and speed.[3] The approach is known as "push-forward combat", which discourages players from taking cover behind obstacles or resting to regain health while playing from the Doom Slayer's perspective.[4]Players instead collect health and armor pick-ups by killing enemies. "Glory kills" is a newly-introduced melee execution system; when enough damage has been dealt to an enemy, the game will highlight it and allow the player to perform a quick and violent melee takedown, rewarding the player with small health recovery.[5]
The fights also require the player to develop simple tactics. As the game gets harder, the player needs to use the advantages of fight arenas and their bonuses smarter. They also need to decide which enemy has to be killed first and how to move on the battlefield.[6][7][8]
The game features a large arsenal of weapons which can be collected and freely switched by players throughout the game and require no reloading. Recurring weapons of the series also make a return, including the super shotgun and BFG 9000. The BFG has a very small ammunition capacity, but is extremely powerful. Similarly, the chainsaw returns, but has been reintroduced as a special-use weapon[9] that relies upon fuel, but can be used to instantly cut through enemies and provide a greater-than-normal drop of ammunition for the player.[10]
Many enemies also return from the original game, such as the Revenant, Pinky, Mancubus, and Cyberdemon, with many also redesigned.[9] Doom's campaign was made to be at least 13 hours long, and the "Ultra-Nightmare" difficulty level features permadeath, which causes the savegame to be lost once the player dies.[11][12] The campaign also features 13 levels.[13]
Many of the levels have multiple pathways and open areas, which allow players to explore and find collectibles and secrets throughout the levels. Many of these collectibles can be used as part of Doom's progression system, including weapon mods, rune powers, and Praetor Suit upgrades. Weapon mods come from field drones and allow the player to unlock alternate modes of fire for many weapons, such as explosive shots and different rate and output of firepower. Each of the weapons' firing modes can be further upgraded using weapon tokens, but they can only be maxed out by completing a challenge related to that particular firing mode. Runes transport the player to a separate arena to perform a combat challenge that grants different abilities when successfully completed, such as better equipment drops from fallen enemies. Players can also upgrade their "Praetor Suit" by retrieving special tokens from dead marines and using them to improve functionality such as equipment, navigation and resistances.[14] Other pickups include small Doomguyfigurines that allow the player to view 3D models of different characters, and data files that expand on the characters and story.[15]
Additionally, each of the game's levels contains a hidden lever which opens an area extracted from a classic level in the original Doom or Doom II. Finding each of these areas unlocks them, making them accessible from the game's main menu in a section called Classic Maps.[13]
