Along with expanding on the refreshing single-player gameplay that originally captivated gamers, Portal 2 will introduce a multi-player element. The much-anticipated puzzle shooter will come with entirely separate levels designed for co-operative action, which puts players in control of "infinitely respawning robots," according to the guys at Joystiq, who got their hands on a preview build at E3.

This makes sense, because the multi-player's higher degree of intricacy causes gamers to die far more often. Not only will there be four portals to manage instead of two, but it'll take more brainpower to wrap your head around the new puzzle mechanics. This added complexity once prompted Valve to say that a multi-player Portal is "less fun than you'd think" - though that was two years ago.

To prevent players from strangling their partners, the developer is implementing a number of communication aids, such as a marking system that gives players the ability to show one another where to stand or where to place a portal. All in all, Joystiq said Portal 2's multi-player aspect is better than expected. The title is due sometime next year for Windows, OS X, Xbox 360, and PS3.