On its face the concept of collectibles in DOOM is counter intuitive. This is a game about combat. Fast combat. Moving and shooting distilled to their essence, then exploding in geysers of demon blood. It’s not uncommon for collectibles to provide some consequential benefit when obtained, but the way in which this has been handled in DOOM feeds back into the core combat loop though multiple overlapping systems. The various collectibles don’t simply encourage exploration of the game’s physical spaces but its systemic ones too.
The collectibles you can locate in DOOM fall into three broad categories. First there are the pure collectibles, the tchotchkes: small portions of classic Doom levels, and UAC MarineGuy toys. Serving primarily as Easter Egg, once collected these allow you to play maps from the previous Doom games, and view in-engine models of the various enemies and weapons.
The second form of collectibles are the Argent Cells, Praetor Tokens, and Field Drones. These are objects that when collected either directly provide, or allow you to purchase, permanent upgrades. Argent Cells will boost your Health, Armour, or Ammunition capacity. Praetor Tokens are used to purchase upgrades for your suit, providing benefits such as reducing the amount of environmental damage you take. Finally, by locating Field Drones you can obtain a weapon modification which grants a secondary fire option that provides additional functionality; two modifications are available for each weapon (excluding the Pistol and Super Shotgun).
The third form of collectibles are the Rune Trials. Each trial leads to a separate challenge level where you are required to perform a set task under specific constraints. If successful you are rewarded with a rune, of which three can eventually be equipped at a time. These runes provide a bonus ability, such as increasing the range at which you can perform a glory kill. Each rune ability can be upgraded by performing a task associated with that ability a given number of times. Upgraded runes provide a more power version of their base ability.
Once first introduced all these collectibles – with a few exceptions – are located only within the secret areas of each level.
Throughout the course of DOOM you can unlock a series of Weapon Upgrade Points, these can be obtained either through combat performance, completing a set of level specific challenges, or locating secret areas. Weapon upgrade points are used to add additional abilities to your weapon modifications, increasing their power and utility. There are between two and three initial upgrades available for each weapon modification, they can be purchased in any order with the cost increasing for each subsequent upgrade. These weapon upgrade points are at the core of where the collectibles feed back into the combat system. Finding all the secrets in a level grants one weapon upgrade point, finding them all of will provide another.
In order to increase your potency and proficiency in combat it is beneficial to equip runes and suit upgrades, the act of locating these also goes toward gaining you a weapon upgrade point. Upgrade points that can’t be used except on weapons you have already modified, for which you will need to have located a Field Drone.
You might not care about collecting all the MarineGuy toys, but if that’s all that stands between you and finding the secrets within a level, and you are one weapon upgrade point short of upgrading your Plasma Rifle, taking a few minutes to find that last MarineGuy is likely time well spent.
Your weapon modifications aren’t just useful for the abilities they provide, like the runes they can also be upgraded through specific use. Once you have obtained all initial upgrades for a weapon modification you have the ability to upgrade it one last time. These final upgrades offer significant improvements, but you cannot even begin progress toward these final upgrades until you have first purchased all previous upgrade levels.
These overlapping systems not only encourage exploration but also experimentation, the abilities available when you have fully upgrade a rune or weapon modification are substantial, such as being able to fire some of the game’s most weapons effectively indefinitely. These can only be obtained from using the weapon modifications and runes in certain ways. Not always in line with your standard approach, the reward for performing the rune and weapon modification specific challenges are potentially worth changing up your play style for. This is also the case with the per-level challenges that provide additional weapon upgrade points. Not only does this type of “get better by doing” approach naturally reinforce the game’s combat systems, it also highlights some of the ways in which these systems can be used that you may not have been aware of, and encourages you to experiment with them.
As you can only equip three runes at once, it behoves you to think carefully about which you want to take. You only gain the abilities of those you have equipped. So, while sticking with three you have fully upgraded makes sense given the strength of their abilities, doing so means you will not be able to upgrade any of the others. Nor take advantage of their abilities. You can personalise and define your own play style based on the runes you equip but swapping them out can lead to interesting systemic interactions. You might want to equip the rune to allow you to engage in a glory kill from a longer range (Seek and Destroy), however if you are taking this it would make sense to also equip the rune that gives you armour from performing glory kills (Armored Offensive). If you are getting armour rapidly you will have an easier time upgrading the rune that requires you to be at full armour (Rich Get Richer). This rune when active means that once you are above 100 armour firing your standard weapon uses no ammunition, the benefits of which are obvious. This rune is especially useful if you are now getting armour from every enemy you glory kill, and potentially picking it up from much further away because of another rune (Vacuum).
Exploring the levels for secrets and investing in upgrading your weapons and runes means that by the closing stages of DOOM you could be wielding a fully upgraded Mobile Turret which can do 660 damage-per-seconds, go through multiple targets, and never overheat (fully upgraded Mobile Turret modification). While also having two chances to come back after death (upgraded Saving Throw rune), be facing enemies that can drop ammunition for your BFG (upgraded Ammo Boost rune), and be able to fire your standard weapon for free if you have over 75 armour (upgraded Rich Get Richer rune). All while taking reduced environmental damage (Hazard Protection suit upgrade), and gaining a full health refill every time you activate a power-up (Healing Power suit upgrade).
The very heart of DOOM is movement and weapons, and though the hunt for secrets can feel slow and incidental doing so will directly improve your combat abilities in dramatic ways.