 Multiplayer mode and player versus player combat were released on July 15, 2008, allowing players to team up and play the game, or fight against each other.A player-run market system (named the Dead Frontier Mega Mall) was created on October 1, 2008, to facilitate trading among players.The barricading system, used to secure buildings, build personal outposts, and prevent zombies from entering them, was released on January 26, 2009.