A developer map called "Zombie_City" that is reportedly created by either Valve or Turtle Rock Studios has leaked online.
The leak of the developer map is part of a larger leak of Counter-Strike maps, and more importantly, Zombie City is a mod for Counter-Strike: Source, which was previously not playable - until now. Notably, Zombie City isn't a fully fleshed-out level as it doesn't actually feature any zombies, but instead, players go up against AI terrorists that can only attack with melee.
Turtle Rock founder Michael Booth discussed the idea behind Zombie City and explained that as the team was developing bots for Counter-Strike: Source, they play-tested levels where developers were armed with automatic weapons and faced off against 30+ knife-wielding terrorists. Booth says that as the team experimented with more prototypes, they kept finding themselves coming back to players versus hordes of AI, or as Booth says, "co-op vs the horde game".
"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," said Booth
Modder WolfCl0ck uploaded the level to Gamebanana on Friday, writing that the mod was likely the starting point for the Left 4 Dead series, and that the mod "upload fixes a texture by adding new material for it. 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."
If you are interested in playing the level for yourself, check out this link here.