Twenty years ago tonight, id Software uploaded Doom to an FTP server at the University of Wisconsin-Madison and completely changed the video gaming industry.
Doom was not the first game to use a first person perspective, nor the first first-person shooter, but it popularized the genre almost overnight, setting the standard so profoundly that for years first-person shooters were routinely referred to as "Doom clones". Contemporary reviewers raved about it, gaming media lavished coverage on it, gamers played it obsessively, it sold millions of copies, and it's left a long legacy to explore, including a notoriously hilarious comic book (RIP AND TEAR!) and equally notoriously awful movie and series of licensed novels. It inspired many notable comments and pushed forward the focus on gaming graphics, as seen in this video promoting Windows 95 as a gaming platform (starring Bill Gates!).
The levels of the original Doom were divided into three "episodes" of nine levels each (levels 1-8 and a secret ninth level accessible via a secret exit from an earlier level in the episode), facilitating the giving away of the first episode, "Knee-Deep in the Dead", as shareware- episodes 2 and 3 being originally available only via mail order direct from id themselves. A later retail release, The Ultimate Doom, added a fourth episode which included some additional functionality developed for Doom II. Doom II, meanwhile, which hit retail shelves a scant 11 months later, added a new weapon and a mob of new enemies, and restructured the levels, having 30 levels plus two Wolfenstein 3D-themed secret levels. In addition to Doom II, iD also released two additional licensed level packs for Doom, the 21-map Master Levels for Doom II and Final Doom, which was comprised of two extremely challenging 32-level .wads: The Plutonia Experiment and TNT: Evilution.
Doom saw a huge number of ports both official and unofficial, to the point that nearly everything runs Doom, including some particularly silly platforms to port Doom to (including Doom 3, of all things). There's even a hardware mod to let you shoot demons with your mind. While most Doom ports were more or less straight ports of the existing levels (with some changes owing to the specifics of various platforms' limitations), the Nintendo 64's Doom 64 in particular stands out. Doom 64, rather than a port of the existing Doom content, was practically a new game inspired by the original- it uses all new sprites for the monsters and weapons, adds a couple of new monsters and a new weapon, and has all new maps and textures. If you haven't played Doom 64, you haven't seen all that Doom has to offer. Fortunately, the Doom64 EX project offers a way to play Doom 64 on PC, so long as you've got a Doom 64 rom.
Perhaps you'd like to take a break from playing Doom to read about it instead. You could start with Tom Hall's Doom Bible, the original design document for Doom, which is perhaps most notable for having virtually nothing to do with the finished product. Then there's Hank Leukart's exhaustive Doom FAQ, included with every copy of the game since 1.4. It lays bare a lot of the underlying mechanics of the game's combat and details all kinds of arcane computer fuckery that used to be necessary. There's also a couple of books on the topic. Most prominent is David Kushner's Masters of Doom: How Two Guys Created an Empire and Transformed Pop Culture, which covers the founding and early days of id Software and focuses on the early corporate culture and conflicts. A more recent effort is Dan Pinchbeck's Doom: Scarydarkfast (at which link you can read the book online), which focuses on an in-depth analysis of the game itself. Or maybe reading's not your thing (who has time, with so much Doom to play?) and you're more into music, in which case you could listen to the Doom and Doom II soundtracks or check out a comparison of Doom's background music to the metal songs they were "inspired" by. Whatever you do, be sure to check out this video of a visit to iD software late in Doom's development.
Okay, let's talk about modding! While FPS modding first emerged as a phenomenon with Apogee's Wolfenstein 3D, Doom was specifically built to enable modding, with all of the data for each game contained in a .wad (Where's All the Data?) file. ( Wikipedia has a great overall article on .wad files, while the Doom Wiki's article on .wad files is more technically oriented. Both are highly excellent.) First, let's take a look at the modding tools available for Doom. If you're interested in modding Doom, you'll want to start with Wikipedia's brief explanation of how Doom levels work, then move on to the Doom Wiki's editing tutorials. You'll need some software to do this with, of course. Make sure you've got a copy of the .wad file for the version of Doom you want to mod (and if you don't, no worries, more on that later), then start downloading some utilities. The biggest name in Doom editing is Doom Builder, a very easy to use level editor that comes with its own set of video tutorials. If you just want to mess around and make some levels, Doom Builder is all you need and you can stop reading this paragraph now. If you want to make your own assets- monsters, weapons, textures, or just random things to litter your levels with- you'll need something more. Fortunately, you have options. The classic tool for DOS back in the day was DeHackEd, the latest version of which is the Windows-based WhackEd 4. Other utilities with similar functionality and different interfaces include the eXtensible Wad Editor and SLADE 3.
But what can you do with these tools? It would be far more work and far more text than is available here to list all of the hundreds of mods and total conversions that have been made for Doom in the last two decades, so here's a sample of the more notable ones.
Possibly the most famous Doom mod ever was the Aliens Total Conversion, which took the frenetic mass slaughter of Doom and made something creepier with it, in the process demonstrating the power and flexibility of Doom and its modding tools.
Cold As Hell takes the action to a deserted Antarctic base and brings a lot of challenge with it.
Brutal Doom starts with the ultraviolence of Doom and takes it way over the top, adding all manner of gore and making the enemies harder.
Action Doom brings the old days of slow-moving bullet projectiles and one-hit kills from 2D shooters to the Doom engine, while Action Doom 2 mixes Doom with classic arcade beat 'em up games.
Knee Deep in ZDoom is a reworking of the first episode of Doom to take advantage of the engine improvements and new features in ZDoom. It's like Black Mesa for Doom.
Deus Vult 2 is renowned for combining excellent mapping with absurd amounts of monsters.
One of the creators of cereal-box prize/Doom mod Chex Quest made a sequel to that legendary advertising vehicle, Chex Quest 3.
Mega Man 8-Bit Deathmatch is a Total Conversion based on multiplayer-focused Skulltag's code which pits the Robot Masters of Mega Man 1-6 against each other.
Call of Dooty is a vicious satire of modern military FPS games built into a Doom .wad. This link goes to the first of a series of YouTube videos featuring a playthrough of the mod; download links are in the description.
When id open sourced Doom, this only applied to the executable code- the art assets and levels (basically the contents of a .wad file) remain id's property. Freedoom is a bit of an odd duck- rather than being a mod proper, which would usually entail needing an official .wad to run, it's actually a GNU-created .wad designed to be used in place of an official Doom .wad, providing a new game which is intended to be compatible with most Doom mods.
Most mods will come with instructions on how to use them- in most cases, just drag the .wad file you want to use onto the executable for your source port, choose the appropriate official .wad that the mod .wad is for, and the source port will handle the rest. (And skip to the end of this post if all this "source port" business is confusing or unfamiliar.)
Every year, the Doom community stalwarts at Doomworld present The Cacowards, a community-curated selection of the best Doom mods released that year. If you're looking for the best that the Doom modding community has to offer, the Cacowards is a great place to start.
For the adventurous (and FTP-client possessing), there's the /idgames archive (the preceding is an FTP link), a massive FTP site of files that has been the major repository for Doom mods over the years. This Doomworld forum post has information on accessing and using the /idgames archive if you are so inclined. If you prefer to simply browse, Doomworld also provides a web-accessible interface to an unofficial copy of the archive here.
Archive.org maintains a huge archive of Doom level packs from CD-ROMS which were sold in stores, such as the famous D!Zone packages. Quality was rarely a concern for the compilers of these quasi-legal collections (nearly all of which were simply collected without permission from internet sources and thrown on a CD-ROM and into a retail box), but if you like Doom and want more levels than you could probably ever play, give 'em a peek.
I made mention of Total Conversions a bit earlier, so if you don't know, a total conversion is a mod that replaces most or all of the assets- the monsters, the weapons, the textures- from the original game in favor of using the underlying engine to make a completely new game. Given the ease of making Doom maps and the low bar for art in the Doom engine, TCs were a pretty common project- well, starting them was, anyway. Here's a huge page full of a ton of total conversions in various states of finished.
If you'd like a peek into Doom history, you can download the original alpha and beta versions (as well as multiple versions of the shareware).
And heck, if all that isn't enough for you? The OBLIGE random map generator will randomly generate playable Doom levels. It's officially impossible to run out.
Doom has also inspired some creations which aren't actually Doom mods. The creators of Classic Doom for Doom 3 took the levels from the original shareware episode of Doom and built them in Doom 3, keeping the layout and enemy placement and upgrading the look to take advantage of Doom 3's much more powerful graphics. Given that the player character was far more powerful relative to the monsters in Doom/Doom II than in 3, Classic Doom for Doom 3 is in many places hard as hell. Moving away from the first-person shooter, DoomRL takes the premise and elements of Doom and makes a fast-paced roguelike. The seemingly dormant Doom: Fall of Mars translates Doom into a Diablo-inspired action-RPG, as odd as that may sound. gmDoom is an addon for Garry's Mod that imports monsters, weapons, and so on from Doom into gMod. A Doom .wad is required. And id's own Doom RPG for iOS makes a turn-based RPG out of the original game- with special appearances from the Wolfenstein dogs.
Let's round this out with the messy business of actually obtaining and playing Doom. Unless you're running DOS or Windows 95, the original Doom software is going to be of little use to you- the code was simply written for systems sufficiently different from modern machines that the original programs mostly don't work, or at best run badly. This is where source ports come in. The short version is that a source port is a program you download that lets you run Doom on your machine even though the original code wasn't written for it.
The first thing you need to do, if you haven't already, is to actually get Doom. id used to sell their games direct from their website, but have stopped offering this service; it appears that the official way to get Doom right now is to buy it on Steam. Each of Doom, Doom II (Master Levels for Doom II is included with purchase of Doom II but has its own Steam page for some reason) and Final Doom is $4.99. Once you've made your purchase and downloaded the game, go into the directory Steam stores it in (search for a folder called steamapps and drill down from there) and grab the .wad file- doom.wad for Doom, doom2.wad for Doom II, tnt.wad and plutonia.wad for Final Doom. Once you've got a copy of the .wad file for the game in question, you can just uninstall Doom via Steam if you prefer- only the .wad data file matters. I recommend moving all the official .wad files you have to a convenient folder on your desktop so you have them handy. If you don't feel like spending money, you can just play the shareware episode (available courtesy of archive.org here- once you've downloaded and installed the file you're looking for is Doom1.wad, and FYI the shareware episode is NOT compatible with most mods) or use Freedoom, though Freedoom will look very different from regular Doom since it's entirely new art assets and maps.
As a quick aside- if you poke around some mod sites, you'll probably come across the terms pwad and iwad. An iwad, or internal .wad, is a .wad file which is the official .wad of a game- doom1.wad, doom.wad, doom2.wad, plutonia,wad, or tnt.wad. A pwad, or patch wad, refers to a .wad which is actually a mod that requires an iwad to run. In general, if it's the .wad from the actual game it's an iwad, and if you downloaded it separately, it's a pwad.
That done, you'll need to choose and download a source port. You have several options for source ports depending on what you want.
Probably the most popular source port is ZDOOM, which in addition to running most any .wads and mods you throw at it also adds a host of new features handily shown off in the Knee Deep in ZDoom mod linked in the list of mods above, as well as support for mouselook, jumping (which WILL break sequence on several levels) and (mostly pointlessly) crouching.
GZDoom is based on ZDOOM but adds support for some pretty OpenGL features.
While most source ports fix the numerous bugs and glitches in the original Doom engine, such as the infamous visplane overflow, the cutely-named Chocolate Doom replicates the vanilla Doom experience, bugs and all, on modern computers.
zDaemon and Zandronum (formerly Skulltag) are the two big source ports for playing online with- a far improvement over the days of IPX and null modems! I honestly don't have enough experience with this to recommend one or the other. Doomworld's front page news frequently announces organized play using both, so if online play interests you, you might want to just grab both.
Your source port's website, or at worst the information that comes with the download, will contain detailed instructions, but in most cases it's as simple as unpacking the download to a folder on your computer, putting a copy of any Doom .wads you have in that folder, and then double-clicking the source port's executable file and choosing the .wad you want to run. To run a mod, you can usually just drag the mod's .wad file and drop it on the executable, choose the correct iwad, and go. If it sounds complicated, don't worry- after the first couple of times it'll feel pretty natural.
You've probably noticed that this post has leaned heavily on two sites in particular: Doomworld and the Doom Wiki. Doom turns twenty tonight, and Doomworld's users have borne most of the burden of keeping the Doom fandom alive and well as the state of the art has moved forward. It's absolutely the hub for Doom on the web, and everybody with an interest in Doom owes them a debt of gratitude for their efforts. The Doom Wiki is a huge and often surprisingly detailed repository of information about Doom and its sequels, and there's many Doom-related topics where the Doom Wiki page is simply the best resource that exists. It's as possible to lose an afternoon wandering its pages as it is with Wikipedia. Without these two resources the state of Doom fandom online would be far poorer and this would be a much thinner post, so thanks to the users and operators of both.