After working on Counter-Strike: Condition Zero under contract for Valve, Turtle Rock Studios had a couple of ideas in the works. One was a first-person wizard game that never got off the ground, while the other was a mod for Counter-Strike that transformed the counter-terrorists into bots who came at you in swarms and only attacked with knives. This was Terror-Strike, and it was the foundation of Left 4 Dead.
As Turtle Rock founder Michael Booth explains in the Left 4 Dead commentary, "While we were developing bots for Counter-Strike: Source we discovered that a few of us armed to the teeth with automatic weaponry against 30 knife-wielding enemy bots was a lot of fun. After shipping Counter-Strike: Source in late 2004 we started experimenting with new game prototypes. That basic kernel of 'small team of friends against hordes of clawing enemies' was something we kept coming back to and we soon realized that the 'co-op vs the horde game' had a ton of potential."
While things like the Special Infected and the personable survivors with their cheery banter came later, the Terror-Strike prototype demonstrates the basic idea's feasibility. Shooting hordes of enemies who don't shoot back actually is a lot of fun. Performing a desperate reload while backpedaling down a hallway as the bad guys cluster and chase you is a frantic good time.
This prototype is part of a larger leak of Counter-Strike map sources. Called "Zombie_City", it may be either the original version of the concept that Turtle Rock showed Valve, or a slightly later iteration. Modder Wolfcl0ck performed a few tweaks to make it playable. "My upload fixes a texture by adding a new material for it", they write. "I have also created an overview texture, generated a nav mesh, and added a level description, all of which are conveniently packed into the bsp itself for maximum convenience."
Zombie_City is available to download from modding community GameBanana. It's a neat look back at the time Turtle Rock made something that impressed Valve so much it acquired the studio, which would eventually go its own way again to develop two more co-op shooters: Evolve and Back 4 Blood.
Thanks to GamesRadar for the heads-up.