A decade ago, from November 17-21 2008, Left 4 Dead was released worldwide on PC and Xbox 360 — and was greeted with the sort of critical reverence to which its publisher, Valve, was accustomed. Principal developer Turtle Rock was entitled to some more genuine surprise. What had started out as a conceptual Counter-Strike mod had impressed the owners of that franchise enough to co-operate on a new, left-field kind of zombie game.
Mike Booth was the keen artificial intelligence maven whose work at Turtle Rock began the journey toward Left 4 Dead. “Looking back, it was a pretty ideal situation," he says, reflecting on the game's origins. "We had built a great relationship with Valve through our Counter-Strike work over the years, and they were in a very strong position with the success of Half-Life 2, The Orange Box, and Steam.” Joint efforts of this scale were rare at the time, according to Booth.
Starting with modified Counter-Strike bots, Booth and his team were working on a zombie game inspired by 28 Days Later’s sprinting undead and, more rosily, the spirit of cooperation. But the game was becoming a “heavy lift” for Turtle Rock, according to Booth. The bigger studio’s experience and resources were therefore welcomed with open arms. At the time Valve’s Doug Lombardi touted the game as a potential Counter-Strike-killer on the competitive scene. In retrospect he’s less bombastic: the game made its mark on players, but didn’t develop a long-lasting competitive space to compare with Valve’s military series. Left 4 Dead was “much more about attempting to create a shared experience” than making a competitive experience, Lombardi now reflects.
Though Left 4 Dead’s gameplay is now familiar, and echoes through games such as Vermintide and Payday, on release it was a completely fresh proposition. Players either teamed up against the AI horde, or formed two teams of four: gun-toting survivors and 'Special Infected'. Survivors were tasked with struggling through short maps to saferooms, intervals of calm and recuperation. Eventually they would earn climactic escapes in delayed helicopters, departing boats and refuelled planes. The opposing players aimed to stop them, with the help of a horde of standard infected enemies controlled by a complex AI that was nicknamed the 'Director'.
This system monitored the level of challenge players were facing — their health bars, ammo counts, progression and countless other factors — to time and place onrushes of enemies for them to cope with. Mike Booth, the architect of the Director, explains that “the ideal situation [...] was the survivors just barely limping their way to the next saferoom, with a horde of zombies closing in.” Before the AI system was in place, Booth adds, players were “just too good at memorising” the campaigns, which “almost turned the game into a race.”
Implementing the Director was, however, a “major calculated risk” for the team. Allowing an AI to control the pacing of a game was nearly unheard of. Reflecting on the development process for the system Booth recalls a period in which “many games would be great fun, [while] others would be really, really easy.” Even more problematically, a game could also randomly turn out as “a brutal meatgrinder” if players were unlucky. This randomness was gradually tuned to more reliably deliver enjoyment – Booth “spent several months in spectator mode watching the rest of the dev team play through the game, monitoring AI Director parameters in realtime, and tweaking variables”. This eventually led to a final system that was “a collection of clever algorithms, tricks, smoke and mirrors, and many, many hours of tuning and balancing.”
Ceding control over pacing to the Director was significant not just for the flow of gameplay, but also for the narratives that Left 4 Dead told – another quietly ambitious facet of its success. The likes of Overwatch and Fortnite are now telling slowly developing stories through environmental details and snippets, but Left 4 Dead trod this path a decade ago. Its narrative lead was Chet Faliszek, then of Valve and now at Bossa Studios, and he still looks back on Left 4 Dead as the “favourite game I’ve worked on.”
Writing narratives for an emergent, randomised multiplayer game was a challenge, but Faliszek recalls that he and his team remained “adamant on the narrative coming out over time, knowing our players weren’t going to just play once.” This allowed them to weave stories in subtly, in the form of rare voice clips, background details and obscure graffiti, in confidence that players would eventually pick up the strands they were dangling. Just what type of “infection” was turning people rabid, what the government was doing about it and how people were surviving all became clearer the more players paid attention.
The resources that Valve brought to the narrative table are evident in the voice work in Left 4 Dead, according to Faliszek: “Valve was invested in having it work, so we simply wrote, recorded, hooked-up, and used more audio lines than most games. That is the brute force method most people overlook.” This uncommonly intensive solution represented the sort of financial commitment that Turtle Rock couldn’t make on its own.
Working with dialogue in innovative and thought-provoking ways is still at the heart of Faliszek’s craft. His work at Bossa has so far been somewhat secretive, but he feels that he is building on the foundations laid in Left 4 Dead. Where that system took “as much data from the world as we could at a precise time” and played the most relevant audio, Faliszek is now working towards having dialogue events feed back into AI systems, to “influence not just future conversations but actual behaviour”. Intriguingly he clarifies that “the game itself is an open world where the player can choose their own path and goals – so the system really needs to be flexible.”
Flexibility was also at the heart of Left 4 Dead’s success, offering players a constant barrage of strategic choices to make. Mike Booth confesses pride at a number of the game’s features, from the way that encounters with the Witch (a rare, weeping enemy with terrifying power) could “usually be entirely avoided” to how “an individual zombie ‘wakes up’ and starts to look around” as “a clear visual cue for the survivors to be extra careful.” These opportunities to avoid combat made it all the more stressful when it began.
As is often the case with such small touches, some decisions that stand the test of time were made on whims, like “making zombies lean into their turn as they sprinted around corners.” After making the tweak himself, Booth recalls that “when we playtested that night and the first horde came sprinting around a corner down the street, it was a 'wow' moment for us. That lean, coupled with the motion capture sprint, made the horde seem much more real, believable, and intense.” This intensity is a huge part of what made Left 4 Dead so memorable, and its cooperative action such a gleeful proposition.
Earlier this year, Turtle Rock advertised a position as Senior Level Designer for a "globally known game franchise", fuelling speculation that Left 4 Dead may be in line for a revival. Lombardi, at Valve, was unforthcoming on the question of the series’s future, and both Mike Booth and Chet Faliszek demurred, but their affection for its first iteration is clear. Spawning a sequel exactly a year later, all the signs pointed towards a series but, as Valve has demonstrated before, finishing trilogies isn’t really the company's speciality.
Left 4 Dead, however, lives on. Faliszek says that he “still play[s] on occasion with modders who continue to mod and add content to the game,” taking advantage of the massive trove of Steam Workshop content available. Mike Booth, meanwhile, is finding a lot to enjoy in the cooperative experiences offered by modern battle royale squad systems, “where the explicitly shrinking zone [...] creates a ton of anticipation and drama as well as great cooperative decision-making.” He hopes “more developers build emergent cooperative game experiences” outside of that genre, and that may well be where Left 4 Dead’s legacy will endure: even if us hard-bitten survivors wait, more in hope than expectation, for the return of the king.