Portal 2 'Design'

Johnson stated that Valve's aim was not to make Portal 2 more difficult than its predecessor, but instead wanted to keep the same idea of a game "where you think your way through particular parts of the level, and feel really smart when you solve it." Portal 2 was designed to give the player incremental steps in understanding portals and their use within the game.This approach led to two basic types of chambers. The first type, which Valve calls "checklisting", provides a relatively safe environment for the player to experiment with a fundamental aspect of a new gameplay concept. The second type of chamber is one that combines these elements in new ways to make the player think laterally, giving the player a rewarding experience for completing the chamber.Chambers were first developed through whiteboard via isometric drawings, with the developers performing a sanity check on the chamber, before being created into simple levels through the Hammer level editor. Extensive playtesting was used to make sure the solutions to each chamber were neither overtly obvious nor difficult to see, and to observe alternative solutions discovered by playtesters; based on their input, the design team would keep these alternate solutions viable within the level, or would work around and block the alternate solutions if they were too easy. Once a chamber was considered ready to proceed, the Valve artists then would add elements such as detailed texturing, dynamic lighting, and vegetation, using an advanced version of the Source engine.  These versions would then be sent back for further playtesting to verify the new elements did not prevent players from finding proper solutions, with further iterations between artists and playtesters until such issues were resolved.

Several early chambers the player experiences in Portal 2 were created by reusing the Portal test chambers, and applying decay, collapse, and overgrowth on them. As an initial goal in the sequel, this was done to give players a sense of nostalgia from the first game and a feeling for how much time has passed. It also allowed the team to avoid the use of the less-resolved textures from the first game, replacing them with higher-resolution dirty and worn-out textures that the newer engine could support. Many of the mid-game puzzles take place in much larger and open areas of the old Aperture Science facilities to make the space "feel epic", according to programmer Jeep Barnett. In these levels, the developers had to ensure that players would hear the dialog lines and could not simply use portals to cross the chamber by making most of the surface unable to accept portals.In the final part of the game, where Wheatley had taken control of Aperture Science, "the level designers just had a blast" in creating the deranged chambers, according to Pinkerton. The level designers recognized that players would become mentally tired of completing a number of similar chambers in a row, and introduced "experiences" for the player every three or so chambers to give the players' mind a rest while expanding on details of the game's story.

P-Body and Atlas

The co-operative gameplay came about from requests from players as well from anecdotes of players working together on the same computer or console to solve the game's puzzles, likened by Wolpaw to players working together on the same computer to solve point-and-click adventure games. The co-operative campaign was also inspired by Valve's Left 4 Dead co-operative games, where players would find enjoyment after playing the game, discussing their personal experiences with the game. While the single player campaign in Portal 2 is designed to avoid frustrating the player, the co-operative levels are more focused on coordination and communication, and are recognized by Valve as being much more difficult than the single-player puzzles. Valve had avoided including timed puzzles into the single player experience in both Portal and Portal 2, but found that the inclusion of these into the co-operative mode was effective, giving players a positive feeling after being able to plan and execute difficult maneuvers. 

Other puzzle elements, such as the light bridges, are further inspirations from Left 4 Dead. Each puzzle chamber in the co-operative mode was assured of requiring four portals to solve as to prevent puzzles being solved by only the actions of one player; as soon as someone discovered a way to complete a puzzle with one set of portals, the level was sent back to the drawing board to correct this. Except in few cases, the chambers were designed where neither player would remain out of sight of the other in order to promote communication and cooperation. Some of the puzzle chambers were designed as asymmetric chambers, where one player would manipulate portals and controls to allow the other player to cross the room, helping to emphasize that the two characters, while working together, are also separate entities. The ability to tag surfaces with instructional icons for one's partner was soon realized as a necessary element, as it was found to be much more effective for cooperation than through simple verbal instruction.