The standalone game studio Turtle Rock, headed by Booth, originally supported Valve in an outsourced capacity. After working on an expansion pack for Valve’s tactical shooter series Counter-Strike, Turtle Rock remained on Valve’s payroll to work on the series’ computer-controlled enemies in its offline, single-player modes, along with a focus on a console-compatible, higher-fidelity version of the game dubbed Counter-Strike: Source. But Valve budgeted additional payments to Booth and Turtle Rock for work that Faliszek describes as “this kind of outside person experimenting and pitching other [video game] projects.”
One of those projects was a zombie-themed offshoot of Counter-Strike, code-named "Terror,” which Faliszek says began life as a mod for the newer CS:S engine. In the mod’s earliest tests, a group of human-controlled players faced off against waves of AI-controlled zombies. "Mike was an AI guy, so this was a logical extension,” Faliszek says. (If you’ve kept up with Left 4 Dead history, you might have seen this initial Terror test in the form of a leaked CS:S map and assets.)
Terror’s gameplay evolved during its pre-Valve prototyping phase until Faliszek and Booth were introduced, at which point the game had settled into something a little more CS-like: four players controlling human survivors with an objective (run all of the way through a map, or set off tanks of gas), versus four players controlling constantly respawning, super-powered zombies, with the latter getting help from waves of weaker, AI-controlled undead.
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