How does a different studio go from handling a slightly less popular edition of the "Call of Duty" franchise (following the massively popular "Call of Duty: Modern Warfare") from hitting their stride with "Call of Duty Zombies"?
Standing their ground and being a little rebellious, that's what. Treyarch executive producer and developer Jason Blundell over on GameSpot outlines the story of how Treyarch made it from there to here—here being "Call of Duty Zombies," basically the existence of the undead in the super-serious military shooters. He credits their willingness to try more daring narratives (like the plotline of "Call of Duty: Black Ops") as the mindset that made them include zombies in a serious World War II game.
The popular game scenario, which began as a custom scenario and Easter Egg from "Call of Duty: World at War," was a long process of love that evolved over the different releases of the franchise.
In the end, it was the fans' acceptance and devotion to "Call of Duty Zombies" that ensured the mode stayed in the "Call of Duty" games. Blundell said that the team would include various, obscure and hard-to-find Easter eggs within the games, but fans were so good (and so numerous) that they always ended up finding what was supposed to be a puzzle. This attitude, however, forces the team to make sure the zombies experience is always a good—meaning, deeply satisfying—experience.
The entire phenomenon has led to the Zombie horde mode being a staple in future "Call of Duty" games—at least, the "Call of Duty" games produced by Treyarch—as it has proven to be hugely popular. The next game in the series, "Call of Duty: Infinite Warfare" developed by Infinity Ward, is confirmed to have the Zombie mode by Polygon, the first "Call of Duty" game made by the studio to have the mode.