When Doom Eternal begins, Doomguy arrives at Earth in one of the most rad spaceships ever: A floating castle called the Fortress of Doom that manipulates space "through an unknown process involving Sentinel energy." In the lore of Doom, it's a command station used by Sentinels for military operations with Argenta design and Maykr technology. More importantly, it's Doomguy's mancave, which contains some of Doom Eternal's best unlockable secrets.
As you discover Sentinel Batteries throughout the game, you can bring them back to the Fortress to unlock various doors to upgrade your character. Unlocking the wrong rooms is perhaps the biggest mistake you can make in the entire game.
Early in Doom Eternal, you'll discover a Sentinel Battery and use it to unlock the Ice Bomb. "Insert Sentinel Batteries into sockets within the Fortress of Doom to unlock items and power up parts of the fortress," the in-game tutorial reads. "Sentinel Batteries can be found in Missions or earned by completing all three of the Mission Challenges."
You should stumble upon many of these Sentinel Batteries naturally in your travels, probably more than you'll truly need, especially if you complete the straightforward Mission Challenges that have you get a certain number of kills, often with a specific weapon against a particular enemy.
The Fortress of Doom has an assortment of collectibles: Mod Bots, Codex Pages, Cheat Codes, Sentinel Crystals, Praetor Suit Tokens, and Skins. Most are locked behind doors requiring two Sentinel Batteries to access, but some are sitting out in the open and are accessed as part of the story.
The in-game map is extremely helpful and shows everything that's available, but don't be tempted by the first doors you find. Some of the best stuff is hidden lower in the ship.
You should prioritize the different collectibles like this:
Sentinel Crystals > Mod Bots > Praetor Suit Tokens > Cheat Codes > Codex Page > Skins
- Purely cosmetic, Skins change the appearance of your Doomguy and don't impact gameplay
- Codex Pages explain the lore of the universe, which is hard to follow as you play
- You can activate Cheat Codes when you replay missions to get infinite lives or ammo, among other things
- Praetor Suit Tokens are the least-vital but still essential currency for upgrading your armor, enhancing your Frag Grenade, Ice Bomb, defenses, map, and mobility. (You can live without these, but they'll make subtle aspects of the game easier.)
- Mod Bots are essential singular upgrades to your guns that appear in most missions. Each weapon has 2 mod slots, and you'll ideally want to unlock all 13.
- The single best upgrades in the game, Sentinel Crystals upgrade your Health, Armor, and Ammo reserves. Once you purchase two adjacent slots, however, you gain some vital perks.
Fortress of Doom Sentinel Battery unlock #1: Sentinel Crystals
You'll unlock the first Fortress of Doom Sentinel Crystal naturally as part of the story. You should spend your first 4 spare Sentinel Batteries opening up the doors leading to the second and third. (See above.)
All of the perks awarded by Sentinel Crystal Routing perks are great. Quickdraw Belch, Napalm Belch, and Belch Armor boost can work in concert to keep you well-armored throughout the game — but you'll need to find a total of six Sentinel Crystal to get there. Loot Magnet is also incredibly helpful. Spend your first Sentinel Crystals working towards these combos.
Fortress of Doom Sentinel Battery unlock #2: Mod Bots
You should be able to unlock most or all of the weapon mods in the game by stumbling upon Mod Bots in the wild, but the two Mod Bots in the Fortress of Doom will help upgrade your weapons by giving them bonus functionality.
Be sure to get Sticky Bombs for the Combat Shotgun, Precision Bolt for the Heavy Cannon, Lock-On Burst for the Rocket Launcher, [General] Improvements for the Super Shotgun, Mobile Turret for the Chaingun, and probably both upgrades for the Ballista. You'll unlock weapons naturally in roughly this order, so depending on how many Sentinel Batteries you have, you might want to stick to this order.
Fortress of Doom Sentinel Battery unlock #3: Praetor Suit Tokens
Once you make it to Praetor Suit Tokens, just follow your heart based on your playstyle, but fully-upgrading either Frag Grenade and/or Ice Bomb should come first, followed by everything in the Fundamentals tree.
To be perfectly honest, none of the other unlockables in the Fortress matter, unless you want to unlock everything.
For an extended look at the Fortress of Doom, IGN has a 13-minute tour: