According to Valve, Left 4 Dead marks the closest collaboration yet between the noted Half-Life developer and another studio. Turtle Rock is essentially creating the game, with Valve helping out at numerous steps along the way and staying heavily involved throughout. Prior to Left 4 Dead, Turtle Rock ported Counter-Strike to Xbox, created the Counter-Strike bot, and developed maps for Counter-Strike: Source. Among Valve's contributions to Left 4 Dead is its writing, which is being handled by Chet Faliszek and Psychonauts co-writer Erik Wolpaw, the pair behind the venerable Old Man Murray.

Even before it was nailed down as a zombie-themed game, Left 4 Dead was conceived as a cooperative game. In each of the four scenarios that will ship with the game at launch, four players must battle through a variety of undead-infested environments to reach some kind of escape craft, such as a helicopter or boat. Left 4 dead is designed through and through to be played by a team of four, so the game will add bots to make up for a lack of human players if necessary. It is even possible to play through the game solo, assisted by three bots, but doing so will drop much of the social interaction and friendly fire-fuelled panic that defines the game as it is meant to be played.

Turtle Rock and Valve plan to support the game after launch with additional full-length free scenarios. By the time the game ships, work will already be well underway on the first of these downloadable scenarios, with the current plan to ship new content this year.

After players have joined a game from a typical server browser, they randomly take on the roles of the game's four visually distinct but gameplay-identical player characters and are deposited into the scenario. At the beginning of each level, and at several checkpoints along the way, players can stock up on new weapons, ammunition, and first aid equipment, all of which soon proves to be extremely useful.

During my play session, we went through an urban scenario, weaving in and out of buildings and through dark alleyways as uncharacteristically speedy zombies appeared, seemingly out of nowhere. Turtle Rock founder Booth has cited Danny Boyle's 28 Days Later, with its fast-moving undead, as his favorite zombie movie, which perhaps goes to explain why Left 4 Dead's infected enemies are more spry than their traditional survival horror counterparts. The zombies' speed ends up fitting the game's format very well. Fast zombies mean that, despite the formidable amount of firepower that can be mustered by four simultaneous players, the game is always able to be frantic and overwhelming. That frantic atmosphere is intensified by the constant possibility of friendly fire, which cannot be disabled.

To keep the game constantly fresh, and to add to its replayability factor, all spawning of enemies is handled by a dynamic "AI director," which acts like a virtual game master. Though the physical layout of the game's scenarios remains constant with each playthrough, enemies will appear in completely different locations at completely different frequencies. During our first run-through, a massive zombie horde was waiting outside the starting building; the next time, we found only a few scattered enemies. The game is constantly evaluating player performance and pace in order to provide an appropriate level of zombie resistance. If your team is struggling, and constantly taking a good deal of damage, the AI director will scale back on the zombie spawns. If your team is chewing through zombies without breaking a sweat, the AI director is much more likely to spawn large hordes and toss in a few bosses.

While Left 4 Dead's standard infected hordes are fairly single-minded, straightforward enemies, its five variety of boss infected exhibit much more variety. The Smoker, reminiscent of the trap-like Barnacles from Valve's own Half-Life games, is able to grab and drag players with his 50-foot tongue. Unlike his Half-Life counterpart, the Smoker can pull his victims in any direction--even down, given certain circumstances--which can be quite startling. The catlike Hunter is able to cloak himself, and can leap incredibly great distances with great speed and agility. The rotund, corpulent Boomer has a surprisingly long-ranged projectile bloody vomit attack which obscures his victim's screen and makes the player the primary target of all nearby zombies. The hulking Tank is simply extremely powerful; his melee attack does massive damage, and he is able to hurl cars and destroy walls, much like a zombie Kool-Aid Man might. Finally, the Witch tends to sit silently unless disturbed, at which point she unleashes hell.

In addition to joining a server as a survivor, players can join as infected, which allows them to take on the role of boss infected--minus the Witch--when the AI director decides to spawn in bosses. Up to four players can join as infected, making for a total of eight player slots per game, in any combination. Players can even join a game solely as infected, letting the AI control the survivors, for a particularly unusual experience.

Left 4 Dead is not a particularly flashy game in terms of presentation. Weapons are functional and basically true to life, but not surprising--there is a shotgun, an assault rifle, a sniper rifle, a pistol (which can be doubled up!), and so on. Thankfully, unlike many shooters, every gun in Left 4 Dead may be used to perform a melee attack. Graphically, the game is competent but not astounding--though Valve staff pointed out that none of the levels have been finalized visually at this stage--with most of the environmental impact coming more from the atmosphere rather than from sheer graphical fidelity. Lights flicker on and off, bosses make creepy noises that alert players to their presence all too late, zombies punch holes through locked doors. Mike Patton of Mr. Bungle and Faith No More, who recently recorded demonic voicework for Starbreeze's The Darkness, is providing the zombie sounds for Left 4 Dead. This all proves extremely effective.

Though Left 4 Dead is very much a twitchy action game at heart, the focus always remains on teamplay. Collapsed wounded teammates can be returned to decent health before they bleed out, if a teammate gives up his health pack and spends several stressful seconds patching things up. First aid kits and quick-fix pain pills can also be given directly from one player to another. With the risk of friendly fire, especially in the game's frequently confined spaces, players must become accustomed to allowing good line of sight to their teammates--when playing Left 4 Dead, Valve employees frequently form up into a rough two-rank formation, with the front line players kneeling to give the rear guard clear shots. Players who die do not simply respawn with the team, but are delivered to safe points and must be "rescued" by their teammates.

Occasionally, that sense of teamplay is necessarily thrown out the window. Upon reaching the end of the scenario, we were forced to survive on a large crate-strewn helicopter pad until the airborne rescue team arrived. Of course, during that nervewracking waiting period, the AI director spawned an absolutely massive, seemingly neverending zombie offensive, complete with numerous bosses and countless zombies. After a while, I found myself the only surviving member of the team, with no practical way to bring my teammates back.

This resulted in one of the most stressful gameplay experiences I have had in recent memory. It was stressful on a whole different level than being overwhelmed in a single-player game, because I knew my three dead teammates were watching me in spectator mode. Valve's Doug Lombardi aptly compared it to being the last remaining member of a team in a game of Counter-Strike. Wolpaw added, "On your end, a lot of that intensity was driven by all those people watching you." Eventually, I was unable to hold back the horde alone and was overwhelmed, though I did pull off some extremely thrilling feats of temporary survival.

Upon ending a level, either by completing it or by failing it, the game presents a breakdown of player performance stats, accompanied by specialized awards or badges of shame for each player. Players are singled out for causing too much friendly fire or for pushing a teammate into danger with a poorly placed melee attack, but so too are they marked for walking into too much friendly fire. More positive awards include recognition for taking out a Smoker before he drags over an ensnared player, taking out a Smoker yourself as he is dragging you over, completing a scenario without dying, completing a scenario with the team intact, pulling off numerous headshots, taking out a Boomer without being attacked by zombies after being vomit-tagged, pulling a teammate up from a ledge, and more. Valve shared little about the planned Xbox 360 version of the game, but it is likely that these awards will tie into the Xbox 360's Achievement system in some way.

With Left 4 Dead, Turtle Rock and Valve are creating an interesting fusion of genuinely tense survival horror and frantic PC-style twitchy FPS gameplay, which succeed in such a novel fashion largely due to the relatively deep teamplay offered by co-op. With co-op play being such an unfortunate rarity in modern gaming, nearly any new example of it is welcome, but one as unusual and fresh as this is all the more so.

Turtle Rock Studios and Valve expect to ship Left 4 Dead for PC this summer, with an Xbox 360 version to follow this fall.