Valve has finally decided to spill some secrets on their highly anticipated game sequel, Portal 2. As dedicated Portal fans remember, the last game ended with the destruction of GLaDOS and presumably of the labs at Aperture Science.

A lot of time has passed since the original Portal, and in this sequel, we see all of the testing chambers have been overgrown and fallen into disrepair over time. We find out that GLaDOS is back from the dead and is rebuilding her test chambers. Portal 2 developer Erik Johnson discussed some of the amazing feedback he received from the first game.

“Our fans said ‘We want more Portal, we want more Portal,’” Johnson repeated enthusiastically. “What we interpreted that as they wanted us to surprise them with the kind of game we’re building again. Which, for game developers, is kind of terrifying.”

Yet, so far, Portal 2 seems to be delivering on what they’ve promised. It’s the same Portal setting and style they love so much,  with new and improved game mechanics that completely change the gameplay. The first surprise that Valve had in store was a new character. Wheatley is a Personality Core, a round robotic eye that is attached to a railing that runs along the ceiling.

At some point in the game, Chell discovers that Wheatley is trapped with her inside the dilapidated labs. He decides to free himself from the railing and swing himself loose. Chell can then carry her cheeky British sidekick around, placing him in door-locks and other enclosures where he can open secret panels and bypass security codes. Wheatley’s presence is refreshing, adding humor and companionship to what is often an alienating game, and offering a dramatic counterpoint to GLaDOS’ wry and threatening dialogue. Not that the companion cube wasn’t a cheery sidekick to have in the original, but Wheatley shows significant promise.

As for the gameplay itself, Portal 2 has a number of new, exciting features. Valve dubbed these game mechanics with names very fitting of Aperture Science — cold, scientific names with a hint of wry humor that masks how dangerous these testing apparati actually are.

For example, the Aerial Faith Plate is a large metal square that you land on that launches you into the air with 5000 lbs of force. The Thermal Discouragement Beam are laser beams that traverse various areas and kill anything that goes through its path. However, this beam can be used to your advantage. Portal 2 has modified companion cubes that are hollowed out in the center so that you can place the cubes in the line of fire and divert the beam in various directions, killing turrets and the like.

There is also the Excursion Funnel, which shoots out a sort of air vent from your portal gun that lets objects float through a tunnel into or out of portals. Finally, there is the Pneumatic Diversity Vent, which is a system of plastic tubes that has a sort of vacuum inside, which you place various objects into — companion cubes, turrets, and the like.

As confusing as it all seems, the gameplay itself is not more complex. “One of the goals for us is to not make the game more difficult or make the game require more traditional action game skills,” Johnson said. “We still want it to be a game you think your way through, so you feel kind of smart as you figure out parts of it. The game’s a lot bigger, so we have more room to ease people in at the right pace, to learn these new mechanics.”

The largest development in the gameplay is the introduction of gels. We were introduced to two different types of gels – repulsion gels and propulsion gels. These blue and orange gels spew out of a pipe or lie in puddles, and the player can use portals to redirect and splatter them all over surfaces. The repulsion gel is a sort of elastic substance that the player can bounce on that flings you and other objects up into the air. The propulsion gel, on the other hand, coats the ground and lets you build momentum to slide swiftly into previously unreachable areas.

Finally, the biggest reveal for this year’s E3, and personally most exciting reveal, is that Portal 2 will have a two-player co-op mode. Valve has made a number of levels specifically designed for two people to navigate together — and with four portals, the test chambers are even more complex.

“A lot of people played Portal 1 as a co-op game,” observed Johnson. “You’re sitting with a friend on the couch, or your parents are watching or kids are watching, so it’s a real natural fit there.” Although not explicitly stated, it seems that multiplayer mode will be both possible for two people next to each other on a couch, or online over Xbox Live and quite possibly the PSN.

Portal 2 will ship simultaneously next year for the PC, Mac, Xbox and PS3.