In context: Over the last several days, a cache of old Counter-Strike content has leaked on the internet. It consists mainly of map files and deleted scenes from Counter-Strike: Condition Zero, which is kind of cool by itself, considering that none of this stuff has been seen by anyone but Valve employees since before 2004.

Perhaps more exciting is that someone uncovered a map in the CS:CZ files called "Zombie City" that was an early prototype for Left 4 Dead dating back to as early as 2004. YouTuber Tyler McVicker notes that despite the name, the mod did not have any zombies. Instead, the proof of concept played out in a cityscape, with up to four counter-terrorist players trying to get to a bomb on the other end of the map (below). However, the real challenge was not to defuse the bomb, but to survive the constantly respawning melee-only terrorists (suitable stand-ins for zombie hordes).

While it doesn't seem much like the resultant 2008 game, Left 4 Dead director and Turtle Rock Studios co-founder Mike Booth confirmed in a May 2022 tweet that the mod was indeed the precursor to the beloved co-op zombie shooter. The Turtle Rock team even tried to pitch the level as an idea for a standalone game, but Gabe Newell wasn't interested.

That didn't stop the studio, though. It eventually moved production to the CS Source engine and codenamed it "Terror Strike." One of the first recognizable assets was a low-resolution character model of a young woman who would eventually become co-protagonist Zoey.

It wasn't until Valve writer Chet Faliszek got wind of the work that Terror Strike began to gain some traction. Faliszek loved the idea and approached Newell about it. The Valve Boss shot him down, saying that zombies were "too cheesy." However, he constantly went on about the game with Newell at every opportunity until he finally caved.

"I was one of the people who checked it out and told Gabe about it at lunch," Faliszek told Kiwi Talkz. "I went on so much about it, he said I should just go work on it."

Faliszek used that go-ahead to recruit more than 100 Valve developers to help with the project in 2008 when Valve acquired Turtle Rock and renamed it Valve South.

Terror Strike launched later the same year as Left 4 Dead to great critical and financial success. The title more than doubled the preorder sales of Valve's The Orange Box, the Half-Life 2 compilation that included standalone games Portal and Team Fortress. Left 4 Dead eventually went on to sell more than 3 million copies.

For those interested and who know how to set up their own CS server, GameBanana has a free, fully intact Zombie City download. Modders have even cleaned it up a bit and squashed some bugs.