Joe Wintergreen is an Australian indie game developer who is currently working on mostly a stealth FPS under the Impromptu Games banner. He also recently delivered unto the internet a series of code snippets from the great FPS Half-Life 2 by way of his Twitter account. There's not much to see of the code, but that's not why we're here. What's really relevant are the code comments, and the light they shine on how the magic is made.

The first tweet in Wintergreen's thread sets the tone for what's to come:

Good to know Valve and I pretty much have the same comments in our AI pic.twitter.com/TrWO8KUAadSeptember 1, 2016

Striders will intentionally shoot things up even if you're not in their line of sight, just because it looks cool:

Strider might still shoot if it knows it won't hit ya if there's something there it'd be cool to hit pic.twitter.com/fHwWO7Cw6CSeptember 1, 2016

Charlie don't surf, and the Combine don't dance:

A fix to make combine soldiers not dance. pic.twitter.com/aXDrhAUk5gSeptember 1, 2016

Zombies had to be toughened up after the shotgun's power was increased, to keep things in balance:

Cool little comment about rebalancing zombies after some changes to the shotgun happened. pic.twitter.com/XxM3ntFm4vSeptember 1, 2016

This is a stupid fix but it works, so whatever:

Might as well continue this thread since I found another good HL2 comment. Feel validated, everyone! pic.twitter.com/gYBCZJJvaRSeptember 2, 2016

There's only one fuck in the entire codebase, according to Wintergreen, and this is it:

Respect to Valve for having only one "fuck" in the entire HL2 codebase. pic.twitter.com/zKNF4ygjIvSeptember 2, 2016

Alyx Vance: Nice girl, handy in a fight, terrible eyesight:

Ah, good bit of HL2 Ep1 commenting (kind of thing they'd put in the dev commentary). On Alyx/barnacles in the dark pic.twitter.com/DhYyooJOyGSeptember 2, 2016

Some of comments really give a sense of what goes into making a game and keeping it intact. One in particular is actually a multi-paragraph telling of how the Strider minigun was accidentally nerfed in the Orange Box because of a bug in the original Half-Life 2 that nobody noticed. There's also a bit about the low violence mode, and references to the Combine Advisors, large sluglike aliens with creepy psychic powers who ended up not being used in the game. [Correction, sort of: I've been reminded that they did appear in Half-Life 2: Episode 2, but you didn't actually fight them.]

It's a fun bit of videogame history, and there are quite a few more than just what's embedded here. If you've got a few minutes, you can catch the entire thread on Twitter.

Every game is ambitious. It s not easy to turn a beautiful idea into a finished, playable game as developers have said time and again, sometimes it feels almost impossible. As miraculous as finishing any game might be, not all games are created equal. Some stretch the boundaries of technology to their breaking point. Others take a leap into the unknown with new design schools, often so effectively that years later, it s hard to remember them ever having to be invented.

Think, for example, of Monkey Island s Three Trials structure, as used by almost every adventure afterwards. Or its sequel s Four Map Pieces , as later picked up by BioWare. And sometimes, both art and science combine to push the envelope and we get something truly, impossibly special. Here are our picks for the top 20 ignoring the very early games that had to prove computers could handle gaming at all.

For more on some of the most monumental games ever to grace the PC, check out our feature on the most important PC games.

King's Quest (1983)

For the longest time, adventure games were where people looked to see the latest innovations. King s Quest set that bar early on, jumping from simple text and pictures to 3D environments, huge worlds, and a fairytale land of mystery to both wander and wonder at. Admittedly, the last part was helped by some dreadful puzzles. King s Quest was originally commissioned by IBM as the showpiece for its long-forgotten PCJr system, but the series would go on to demonstrate just about every major technological advancement for the mainstream: ADLIB sound, VGA graphics, full speech, and high resolution. 3D didn t work out so well, but until that point, King s Quest was where many players went to get their glimpse of the ever-advancing future.

Commander Keen (1990)

If you want to experience pure hell, try the average 80s PC platform game. Long before making Doom, the team that would be id Software wanted to prove that the PC could handle experiences that played as smoothly as dedicated consoles. Commander Keen wasn t just a fluid experience by the standards of the time, but a fast one, with pogo-jumping, shooting and big levels to explore. Looking back, it s hard to appreciate what a development it was, but we re talking an era where games like the original Duke Nukem (or Nukum either way, the one who wore a pink suit and watched Oprah) were constantly being held up as the PC s answer to Mario. Commander Keen didn t qualify either, but it paved the way for many sequels and the formation of id itself.

A bit of bonus ambition: before making Keen, id tried to convince Nintendo to let it port Super Mario Bros. 3 to the PC by building a working demo (in their off hours in a single week, no less). Nintendo said no thanks, but you can see footage of the demo here.

Maniac Mansion (1987)

If you made a game like Maniac Mansion right now, people would still rightly call it ambitious. A choice of seven characters, each with their own skills. A non-linear adventure with five different endings depending on choices and characters. Real time elements, like ringing the doorbell and having a character come downstairs to check on it. Puzzles involving multiple characters in different rooms of the house or simply the option to do things like put a kid in an empty swimming pool and then fill it back up. And on top of all of this, Maniac Mansion brought the world the SCUMM system (Script Creation Utility For Maniac Mansion) that would define about half the adventure game market for the next decade. All of this, in 1987. Few adventures have ever done so much.

Ultima Underworld (1993)

Like most of the games on this list, Ultima Underworld is a fusion between ambitious technology and ambitious design the design side specifically being to take one single dungeon and try to breathe life into it. To add nuance to its different races, there to be talked to instead of just beaten up. The Stygian Abyss wasn t just a battlefield. It was a fallen community. A place to live in. The experience of being thrown into a dungeon and just expected to survive.

What really sold it though, if your PC could run it, was the technology. Before even Wolfenstein 3D, Ultima Underworld offered a full 3D environment complete with slopes, lighting effects and more, in a bit of technology that could only have been more impressive if well, the viewing window had been a bit bigger. Underworld 2 greatly increased the scope of the game, visiting other worlds and making it a bit easier to see, but what the first one managed remains a technological victory worthy of any heroic age.

Ultima VII: The Black Gate (1993)

Get used to seeing the word Ultima. Ultima VII came out in 1993, and still games like Divinity: Original Sin measure themselves against its success. Its biggest success was creating a living world, where peasants went home at night, weather blasted the world, your companions had to be fed, and, yes, where you could get some flour and water, mix it into dough, stick it in an oven, and get your own deliciously crispy bread. On top of this was an incredibly mature story that continued the series love of more advanced storytelling than most games of the era (previous ones having tackled racism, the perversion of good, and the quest for a hero worth being called one) with a complex tale of good intentions subverted by an otherworldly being of pure, but incredibly smug malevolence.

Ultima Online (1997)

Last time! Where Ultima VII brought a living world to single-player RPGs, Ultima Online brought it to multiplayer. It wasn t the first MUD or MMO, but most of them followed the Diku model popularised by Everquest: go forth, slay. Ultima Online wanted to create an actual world, where players would gather resources, craft houses, become shopkeepers and more, with hero just one of the many careers available. It wasn t without its problems, the first of them being the discovery that given a world to explore and exploit, players will typically turn it into a survival of the fittest Hell. But, its scope, its potential, and the joy of it when it worked created an epic experience that s still running today, and stories like the assassination of Lord British that will never cease to amuse.

The Elder Scrolls II: Daggerfall (1996)

The second of the Elder Scrolls games asked one hell of a question: could you make a world with over 750,000 characters and a map the size of Britain actually feel like a world? We re putting this one here instead of Elite, partly to ring the changes, but mostly because few procedural games have pulled it off so well enough political relationships, guilds, interesting stuff to discover, and cool mechanics like being able to get turned into a werewolf or vampire.

It s not that difficult to create raw space. Daggerfall s own predecessor Arena offered even more. Its sequel, Morrowind, did what most games tend to, and hand-crafted a far smaller area in intricate detail. But for a moment with Daggerfall, we had a game that showed you could be epic, procedural and interesting, without simplifying everything down to the ASCII style of Rogue or putting all the impetus on the player to pretend that there was more going on behind the surface than was ever going to meet the eye.

Duke Nukem 3D (1996)

While another case of a game that s not aged all that well, Duke Nukem 3D was the game that took FPS action out of military bases and sewers and relocated it to city streets, cinemas, and other more realistic locations. That plus a complicated scripting system to blow them all up, clever tricks to fake a 3D engine (even though it was only 2.5, much like Doom) and endless imagination took Duke from being a moderate shareware star to the highest tiers of game characters. No wonder the world was willing to wait so long for Duke Nukem Forever. Even if it wasn t worth it, in the end.

Outcast (1999)

The PC has never really had its own Legend of Zelda. Action. Exploration. A whole new world to explore. Outcast is arguably the closest its come.

A graphical powerhouse of a game that immediately impressed with its freedom, with the AI of its characters, with the glorious effects in everything from jumping into water, to your personal scanner rippling gridmarks across the scenery. There was only one problem. It was all done with voxels at a time when 3D cards were finally allowing for decent polygonal worlds, putting all the work on CPUs that couldn t handle it. If you could play it, Outcast was an unforgettable experience. Too bad for most people it was one that had to wait until the GOG version that finally made it run, long after its prime.

Grand Theft Auto 3 (2001)

It s easy to dismiss the sheer effort that goes into creating a city. After all, we ve walked, run, driven and carjacked around so many. GTA 3 wasn t even the first, with racing games in particular having set the pace. But could you get out of the racing cars and ramble? Enjoy a pumping gangster soundtrack? Run around with automatic weapons and go on missions with a huge cast of crazy characters? Just sit back and listen to an hour of talk radio? Nope. GTA III was magic, and so many sequels on, it s still raising the bar for what virtual cities can and should be.

Give or take a few terrible cock jokes, anyway.

EVE Online (2003)

Ultima Online intended to let players call the shots. It didn t quite work. With EVE Online however, CCP had the courage to actually let it happen, creating one of the most talked about online games of the last few years. Tales of empires at war, of con artistry on a scale that would make Count Lustig blink, the epic sagas of backstabbings and betrayals that no other game can match. CCP likes to describe EVE using the phrase EVE is Real , and while there may not be any starships flying distant galaxies under your favourite forum s command, they still have a point.

Civilization (1991)

All of human history in a single game? There s not much more to be said, really. As achievements go, the only bigger one would be making it one of the greatest games of all time. Not to cast aspersions on the likes of Elite for creating a universe in slightly fewer bytes than the average person would make in a toothpaste and peperami footlong, but the thing about space is that it is mostly empty. Just saying. The world however, in as many ways as you can imagine? That s ambition, even if using it educationally does mostly teach people never, ever to mess with Gandhi.

Deus Ex (2000)

Real world. Real conspiracies. Where do we even begin? Deus Ex not only set out to create some of the most realistic real-world locations we d ever seen (not a tautology the games before hadn t exactly done a great job most of the time), but also turn them into nothing short of a psychopath s toolbox. Multiple paths and solutions. Characters who reacted to your decisions. Tiny decisions determining who lives and who dies. All wrapped in some of the best writing and wrapping the PC had known up to this point. There s a reason why so many years on, it s the original Deus Ex that still stands out as both one of the greatest games ever, and the template of a dream for future immersive simulators to study at the feet of as they try to surpass it.

Dwarf Fortress (2006)

Simulations don t get any deeper than this. Literally, or figuratively. Dwarf Fortress or to give it its full title, SLAVES TO ARMOK: GOOD OF BLOOD, CHAPTER II: DWARF FORTRESS is an ASCII gem best summed up by its creator saying in 2011 that we shouldn t expect version 1.0 for at least twenty years. That s what you get in a game so crazily detailed that a cat can go into a tavern, pick up spilled alcohol on its paws, wash itself off, and get drunk. This was never intended behaviour, just the sum of smaller sub-routines coming together and making their own reality. In retrospect, that twenty years to complete doesn t sound so much at all.

Half-Life 2 (2004)

In a way, Half-Life 2 s most ambitious part isn t even in the game. Valve had an idea for a new store, called Steam . You might have heard of it. Half-Life 2 was, if not its Trojan horse, then its vanguard. You wanted to play the best FPS ever made at the time? Then you got it through Steam. And that worked out pretty well.

Even if you ignore Steam, Half-Life 2 reinvented the shooter with its focus on physics, with every chapter introducing new mechanics and new exciting concepts like the gravity gun or playing point-defense with turrets. It also created a continuous world like no other, putting the final nail into the coffin of games that didn t prize a sense of presence as well as place in their shooter campaigns. Much copied, but still rarely bettered, Half-Life 2 set out to be both the best shooter around, and its next great leap forwards.

Planetside (2003)

Some games just shouldn t be possible. Even knowing the technology that powers them, the epic battles of the Planetside series have always had a degree of magic to them. For the handful of players lucky enough to have a system and connection that could handle it, heading out into one of Planetside s huge battles is a defining moment in games. For the rest, it says a lot that it still felt just as impressive when Planetside 2 rolled along only a couple of years ago. 5v5? 12v12? That s all well and good. But an explosive, expanding, all-access battlefield where the war never stops? That s military action with a little sorcery mixed into the formula, even today.

Spore (2008)

It failed. Yes, we know. It failed. But this is ambitious games we re talking about, and few games shot higher than Spore. Leading a tiny organism through every stage of life. Constructing it using the surprisingly powerful and fun editor. Sending it out to meet other players aliens in a great throbbing galaxy full of freshly created life. That may have been the point where the charm ran out, but the open-ended action and procedural generation and early focus on user generated content that led up to that point still stands out as a technological, if not gaming success.

Crysis (2007)

"But can it run Crysis?" was a relevant joke among PC gamers for at least three years for good reason: well after 2007, Crytek's shooter could still bring CPUs and graphics cards to their knees. Crysis took Half-Life 2's early use of physics and applied it to a dense, free-roaming world. Being able to shoot a tree, watch it fall over, and then shoot the trunk into smaller pieces was revelatory players gladly gave up framerate in favor of insane graphics and physics processing. Cutting edge AI and the systems-driven sandbox gave Crysis the depth to match its insane graphics, and no shooter since has managed quite the same combination of wow and substance.

DayZ (2012)

From its beginnings as a popular mod, DayZ spawned one of the most popular genres in gaming today. The framework for this multiplayer zombie survival game was Arma 2, up until that point one of the most ambitious simulation games and a bastion for fidelity and scale on PC. DayZ built upon Arma 2 s ambition, borrowing and later adapting its 225 km2 terrain, Chernarus, which was created from satellite-modeled slices of the Czech Republic.

The month that DayZ caught on, creator Dean Hall was already laying out incredible plans about features he wanted to add, as he told us in an interview. Underground structures. Dog companions. Realistic disease systems. A couple months later we were hearing about destructible terrain and player cities. Part of Hall s stated approach was to experiment with big, bad ideas, but the reality of implementing them quickly in Arma s Real Virtuality engine for DayZ proved to be a massive challenge.

Outside of these early technical roadblocks, as a multiplayer game DayZ was uniquely trusting. The systems that DayZ inherited from Arma granted it some depth, and being dropped into a massive, hostile environment with no instruction empowered players to tell their own stories, often through surprising, weird interactions with other survivors.

Our 2012 mod of the year remains in Early Access after arriving on Steam as a standalone game on December 2013.

The Witcher 3: Wild Hunt (2015)

It s amazing to think that in just three games, CD Projekt Red has gone from unknown studio to absolute top-tier RPG developer. The Witcher 3 is their masterpiece, from the hand-crafted world to the sheer number of characters and plots. It s a game that excels on every level, from scripting subtle enough for a character to break off combat when they hear your name, to the global nature of some of the most amazing graphics and scenery in any PC game ever, and the sheer artistry of just about every major quest or aside. You never know what s coming next, from the teary humanity of the Bloody Baron s agonising storyline, to a gaggle of Witchers drinking too much, dressing up in drag, and drunk-dialling wizards across the whole continent.

Star Citizen (20XX?)

No, it s not out yet. It doesn t matter. Chris Roberts play to create the ultimate space game already qualifies. Elite style action combined with a dedicated, AAA Wing Commander-style campaign starring Mark Hamill. First person action aboard ships. Deep space exploration. A persistent universe allowing for company, or the solitude of the stars. A crowd-funded budget of $117,259,371 and counting, with players happily putting down real money for in-game ships and unlocking features like pets and modular ship designs and new AI characters to scatter around on planetside environments. If it s not the greatest game ever, expect literal, physical riots.

Modding Half-Life 2 is still an enriching pastime for thousands, but did you know you can mod life to be more like Half-Life 2? Yeah, it's a thing: Russian handyman Valplushka has fitted a remote control drone with all the adornments needed to make an authentic Half-Life City Scanner drone.

It's a pretty impressive setup: the front panel moves emotively, just like in the game, and there's a red laser light embedded inside. The mind boggles at the potential. I'd like to have one as a pet, actually.

The first video is a simple demonstration of the drone, while the following one gives you some idea of how to make your own. Cheers, Geek.com.

I can't remember how long it took me to finish Half-Life 2. It's been a while, after all. But I can say, with absolute confidence, that it was a lot longer than the breathtaking 40:49 it took the SourceRuns Team to do it.

As is usual with speedruns, this is not a typical playthrough of Half-Life 2. It's done using a 2006 build of the game that used the original engine, which has significant movement differences, as well as a long list tricks and glitches that have since been patched out. It's also segmented, which basically means that it's a group effort: Different players hammer through different bits of the game, and the best of them are stitched together into what you see in the video.

The net result is fast, furious, and funky, as the runners clip through walls, fly over levels, and blow past the talkie bits. It doesn't look like much fun in the conventional videogame sense, but it's a hell of a sightseeing tour, and a remarkable accomplishment, coming in at just one-third of the world-record mark the SourceRuns team set in 2013.

A spreadsheet that breaks down just about every element of the run you can imagine is up on Google Docs, and the SourceRunners have also posted a separate video explaining how clipping works on YouTube.

Half-Life 2 turns 12 this year, and thanks to its powerful, if a bit creaky Source engine it remains as popular with the modding community as ever. Over the years we've seen all manner of excellent mods emerge, adding co-op or competitive multiplayer, shiny graphical updates, new story content, and even full conversions that bear little or no resemblance to the original game.

It's the latter two we're going to focus on today, as we round up the best single-player Half-Life 2 mods. We've chosen mods that stand up as separate adventures, sometimes set in worlds far removed from Combine Earth.

The Stanley Parable

This is the story of a man named Stanley. Or rather, it's the story of the story: a deviously clever, reactive adventure that second-guesses your every move. As Stanley or, perhaps more accurately, as the player controlling Stanley you're free to follow or ignore the various instructions the wonderful narrator bellows over you, resulting in a tangled, branching story that rewards your curiosity, imagination, and defiance. The original Source mod was later expanded into a full game, one our Phil thought extremely highly of in our review.

Download: ModDB, Steam.

Minerva: Metastasis

Adam Foster's Minerva comes close to the quality of Valve's own Half-Life 2 Episodes in fact, Valve was so impressed Foster joined the company. It's a sizeable story, about the length of an official chapter, with considered level design and a high level of polish. You begin the game strapped to the underside of a helicopter, before being dropped on a mysterious island with a sinister secret.

Download: Steam.

The Citizen

Gordon Freeman ends the Half-Life series as a crowbar-wielding superhero, a figure of legend in the Half-Life universe. Two-part mod The Citizen provides a new angle on the world, casting you as an ordinary oppressed citizen of City 17. Obviously, said ordinary man soon acquires a gun and starts killing people, but you might snap too if you called that dystopia home.

Download: ModDB.

Get a Life

This lengthy, ambitious mod swings from horror to all-out action. Occasional cutscenes tell the story of a subway technician suffering from leukaemia, but Get a Life's unlucky hero Alex also has to contend with the mod's new limb damage system, which causes effects like dizziness and limping, depending on where he's hit by enemies.

Download: ModDB.

Mission Improbable

Your mission, should you choose to accept it, is to step into the sneakers of Gordon Freeman and set off to repair a Resistance listening post. This impressive Episode Two mod begins with Gordon rowing to a distant coastline: a coastline that reminds you just how pretty the venerable Source engine can look in the right hands. The right hands in this instance are a couple of established game devs, and their experience shines through pretty much every crevice of this slick, well-paced adventure.

Download: ModDB.

Research and Development

Thanks to its then-revolutionary ragdoll physics, a lot of time in Half-Life 2 was spent throwing chairs at NPCs, or flinging teacups with the gravity gun. In that spirit, Research and Development does away with offensive weapons altogether, leaving just a couple of secondary tools to let you manipulate gravity or order Antlions about. Puzzles are the order of the day here, and it's surprising just how easily Half-Life 2's toolset translates to this new focus.

Download: ModDB.

Nightmare House 2

Where there are modding tools, horror mods are sure to follow. You don't need to have played the original in fact, it's included as a prologue, giving you the chance to explore both a haunted house and a spooky hospital. The horror on offer here is mainly of the jump scare variety, so if you were hoping for the psychological horror of Silent Hill, move on to the next item in the list. Nightmare House 2 is basically FEAR it even features its own creepy ghost girl but more FEAR is hardly a bad thing.

Download: ModDB.

Silent Hill: Alchemilla

The impressive Alchemilla drops you in the world of Silent Hill, endless fog, Dark World and all. Not only have the developers nailed the grimy aesthetic of Team Silent's classic series, they've matched its colour palette, borrowed its sound effects, and recreated its lonely atmosphere. It's such an uncanny representation that it may take you a while to notice there are no enemies traipsing around, but then those games were hardly known for their satisfying combat.

Download: Alchemilla mod.

Water

Until now everything we've featured has been strictly first-person, but Water bucks that trend. In fact, it bucks a lot of trends, given that it's a third-person puzzley adventure starring a mermaid. Yes, a mermaid. While you're (initially at least) limited to a fantasy city's waterways, this smart mod soon finds ways to get you exploring land too, using a number of innovative systems. The developers of Water went on to make From Earth, another, similarly inventive Source mod.

Download: ModDB.

Black Mesa

Well, we couldn't ignore Black Mesa, could we? For the unaware, this recreates the original Half-Life in its sequel's shinier engine, and it's been in development since dinosaurs roamed the Earth. Much more than a simple port, Black Mesa updates Valve's game with fancier assets, new voice acting, a reworked campaign and more. The team have also excised Half-Life's much-maligned Xen section, albeit only temporarily until it's been remade to be, somehow, good. While the older mod is free, you'll find the newer version on Early Access, accompanied by a price tag.

Download: Official site.

Push Half-Life 3 to the back of your mind, where it surely spends most of its time these days. How about some new Half-Life 2? Indie dev Richard Seabrook has spent the past two years working on a continuation of Gearbox's Half-Life 1: Opposing Force story, and the result, Prospekt, is said to match Half-Life 2: Episode One in length. February 11 is the big day.

At one point in Prospekt's development, Seabrook loaded it onto memory sticks, packed them in a briefcase adorned with the lambda logo and dispatched it to Gaben. He did not hear back. After passing through Greenlight, however, Prospekt gained Valve's nod of approval for the licence and assets.

As Gordon Freeman is cornered in Nova Prospekt, the Vortigaunts teleport US Marine Adrian Shepherd into the fray to give him a fighting chance. That's you. In total, Prospekt has 13 levels (including a return to Half-Life 1's Xen, which the long-running Black Mesa project is still working on) and upgrades the visuals of the original setting.

Prospekt will cost 7.50/$10. You don't need to own Half-Life 2 to play, but I'm saying that you should own Half-Life 2, you strange maverick.

Pining for Half-Life 2: Episode 3 is so passe. I'm more interested in Episode 4, which was being developed by Arkane until it was killed for being too good for this world. Or, for sensible reasons as revealed by Valve's Marc Laidlaw three years ago. It was to return players to Ravenholm, hence the name 'Return to Ravenholm', and some legit-looking screenshots seemed to bear that out.

I'm not just bringing this up to open old wounds—some new screenshots have been unearthed courtesy of those scamps at ValveTime. There are 11 of the beggars, purportedly taken from the portfolio of one Robert Wilinski, a Senior Environment Artist who was at Arkane between 2007 and 2008. You've already seen one above, but here are a few extra of the more interesting ones. (Having said that, they're all a bit boring—damned sewer levels.) Click this link, or watch the following video, for the others.

There are also a couple of images reportedly of Arkane's similarly cancelled The Crossing. It wasn't Half-Life related, but at least it wasn't set in a bloody sewer.