Fortress of Doom (Doom Eternal)

The Fortress of Doom is a massive Sentinel space station, serving as a central hub and player base in Doom Eternal. It offers an in-game setting to display the Easter egg items collected by the player, unlock various items, as well as practice safe fights in the "Ripatorium".

The player can obtain several weapons in the Fortress of Doom over the course of the game's story after specific missions, the flame belch is introduced after Hell on Earth, the ice bomb is introduced after Exultia, the ballista is obtained after Doom Hunter Base and the Unmaykr is unlocked after obtaining six Empyrean Keys as early as after Taras Nabad. All of these weapons (barring the Unmaykr) are required for the story to progress.

Once the player has set their destination to Nekravol, they will be warned that they cannot return to the Fortress of Doom until the end of the game.

Lore
The Fortress of Doom is the Doom Slayer's combined spaceship and base of operations. It was originally a Night Sentinel command station, built during the reign of King Danak. Despite the close involvement of the Maykrs in its construction, its primary systems appear to have been specifically intended for Argenta use. Regardless, the Khan Maykr still has influence over some of its subsystems, as can be seen when she remotely deactivates its power supply (later replaced with the demonic Crucible by the Doom Slayer, to the bemusement of Samuel Hayden). The fortress has no propulsion systems, and instead uses an unknown process utilizing Sentinel energy to manipulate space around itself in order to move.

According to VEGA, the fortress is likely to be the last trace of what was once a much larger fleet.

Easter eggs
The portrait of the Doom Slayer holding his pet bunny Daisy was originally a piece of fan art created by Ian Nielsen (Xous) and placed into the game after an initiative from id Software's Kathleen Tigue.

The two original Doom games are both playable on the computer: Doom is unlocked by collecting all 14 cheat code diskettes in the main game (two are hidden in the fortress itself), and Doom II is unlocked by entering the password "FLYNNTAGGART" when prompted.

The skateboard is a reference to Doomguy's presence in  as an unlockable character.

Bookshelf
The Doom Slayer's library is filled with Doom-themed parodies of classic literature (most of them simply shoehorning "slay", "rip and tear", "demon", "guts", or a level name from Doom (2016) in the title) and references to other video games.

Literature:
 * Dungeons & Demons - 6th Edition: . As of Doom Eternal's publishing, the fifth edition of that tabletop game is the last.
 * The Count of Kadingir Cristo:  by.
 * Eat. Rip. Tear.:  by.
 * Fifty Shades of Slay:  by.
 * Slayenstein:  by.
 * Don Slayote: , by.
 * Slayerhouse Five: , by.
 * My Best Fiend, Daisy: This one might not be a reference, and just a book written by the Slayer himself.
 * The Ripping Tree: ', by or ', by.
 * The Guts of Wrath: , by.
 * The Man in the High Argent Tower: , by.
 * How to Stop Worrying and Start Slaying: , by.
 * The Slayer's Tale: , by.
 * The Art of Rip and Tear: , by.
 * The Power of Positive Ripping and Tearing: , by.
 * The Very Hungry Cacodemon: , by.
 * Demon Farm: ', by '.
 * The Great Gutsby: , by.
 * To Kill a Mockingdemon: , by.
 * 1984 Dead Demons: , by George Orwell.
 * Green Eggs and Pentagram: , by.
 * The Caco in the Rye: , by.
 * The Picture of Dorian Slay: , by.
 * Atlas Ripped and Teared: , by.
 * How to Win Friends and Kill Demons: , by Dale Carnegie.

Video games:
 * My Buddy Superfly by Hiro Miyamoto: . The protagonist of this game was named Hiro Miyamoto. One of the NPC teammates was called Superfly Johnson. The phrase "I can't leave without my buddy Superfly", which was used to tell the player they had to make sure their AI companions were nearby before exiting the level, became a meme. The name of this character was, in turn, a reference to an old Doom fansite, "Abode of the Superfly".
 * Why I'm So Great Pt. II by Dork Norkem: Duke Nukem series. Duke Nukem II started with Duke in a TV show, presenting his new book "Why I'm So Great".
 * Devilish Daggers & Other Assorted Pointy Objects: . An indie video game released the same year as Doom (2016), with a retro aesthetic and gameplay inspired by Doom and Quake.
 * LIANDRI - A Brief History of Interplanetary Industry: . The Tournament is organized by the Liandri Mining Corporation.
 * Von Braun - Onboard AI System Technical Manual: . The Von Braun is the spaceship in which the game starts. Its AI, named Xerxes, was taken over by enemies, and shutting down Xerxes is a major goal in the game.
 * Mesa Science Monthly - Predicting Unforeseen Consequences: . The game is set in the Black Mesa Research Facility. "Unforeseen Consequences" is the name of the first "real" level of the game, after the introduction part is over . In addition, it appears as a phrase in later sequels.
 * U-NAT-CO Training Manual: Bomb Defusal: . The UNATCO (acronym for "United Nations Anti-Terrorist COalition") is the organization the protagonist, JC Denton, works for at the start of the game. The reaction of the protagonist when a bomb is found in a helicopter has become a meme.
 * The Spear of Destiny: A Post-War Deconstruction: Wolfenstein 3D. The commercial game made with the same engine and mostly the same assets is entitled Spear of Destiny.
 * Living With Rage - and Other Common Emotions in the Apocalyptic Wasteland: . The game does take place in an apocalyptic wasteland.
 * THE STROGG: A Transdimensional Field Study: . The Strogg are the enemies in that game, an empire of evil cyborgs invading other planets with teleportation technology to stripmine them of resources and forcibly converting their denizens into more mind-controlled cyborgs for their armies. The BFG-10000 also resembles the Strogg's Big Gun weapon system.
 * RET-CONNED: The Life and Times of Flynn Taggart: Doom novels. The main protagonist in the official novelization of the classic Doom games is called Flynn Taggart. The story in these novels rapidly diverges greatly from the game, however, effectively retconning everything. "FLYNNTAGGART" is also the password to unlock Doom II as mentioned above.
 * From Nopefish to Dopefish: The Dope Tale: Commander Keen IV. The Dopefish from that game, originally described as a ravenous idiot, has managed to find its way into cameo appearances into many other games: a true success story.
 * Tei Tenga - Offworld Travel Guide: Doom Bible. The fictional planet of Tei Tenga was the originally-planned setting for Doom, before the action was moved to the moons of Mars.
 * Cooking from Hell's Kitchen - Thy Flesh Consumed. The Ultimate Doom. Thy Flesh Consumed is the name of the fourth episode added to Doom for its retail release. In addition,  is the title of a reality television show which involves cooking.
 * 2 Prey or Not 2 Prey. . This game had, long in development hell, that was ultimately canceled by Bethesda. An was later released by a different studio, simply titled Prey rather than Prey 2.
 * Vault Dweller's Survival Guide - Preparing Yourself and Your Loved Ones For the Inevitable Nuclear Apocalypse. . The setting's premise is that just before a devastating World War III, many people were selected to take refuge in secure underground shelters, called "Vaults". These people and their descendants are referred to as "Vault Dwellers".
 *  Nuka-Cola - A History from the Atomic Age of Flavor. Fallout series. Nuka-Cola is the number-one brand of soft drinks in this setting.

In addition to the jokes, there is one large book entitled "how to comb your mustache" by "Clifton M. Fischbach". This is actually a homage to the late father of popular YouTuber.

Commander Keen's skull, helmet, and neural stunner are stored ontop of the bookshelf. The lower shelves contain Slayer-branded game consoles and several classic id Software game boxes, including the original Doom and Doom II, Quake, and Quake III: Arena.

Records
Sixteen vinyl records are hidden throughout the levels. Finding them adds a corresponding poster to the Fortress of Doom. The records are of songs from various id Software games.

Demon toys
Demon toys can be collected throughout the levels. Activating one of the demon toys allows to open the model viewer for the monster in question.

Collectibles
The Fortress of Doom contains several collectible items. In total, there is one codex page, two cheat codes, two modbots, five Praetor suit tokens, three sentinel crystals, and the Unmaykr.
 * After Hell on Earth, one sentinel crystal is available on the bridge.
 * After Exultia, the "Fortress of Doom" codex page and one Praetor token become obtainable.
 * After Doom Hunter Base, a number of other items become unlockable, requiring two sentinel batteries each. These items include the "all runes" cheat code, the two modbots as well as the remaining sentinel crystals and praetor tokens. In total, eighteen batteries are required to unlock them all.
 * After Sentinel Prime, the "fully upgraded suit" cheat code becomes available.
 * After Taras Nabad, if all six Empyrean Keys have been collected from the various Slayer Gates, the Unmaykr can be unlocked.