One of the commentary nodes by a Portal 2 developer, Eric Tams, said they used “world portals” to quickly reconfigure maps and puzzles during testing, which allowed them to create rooms that were bigger on the inside, infinite falls, and other sorts of impossible geometry, save for the users own ASHPD space bending.
When we started the project, making any big structural change in a level or the order of levels would lead to hours or even days of busy work trying to reconnect things and make sure they lined up again. If we ever wanted to ship something the size of Portal with the finely tuned balance we desired then we needed a way to be able to make big changes to the layout of the game without paying the cost of making everything line up again. We needed a way to bend space.
Soon every connection between any space was a portal. We would even switch them on the fly. Even a simple door worked like the cartoons – just a facade painted on a wall that seamlessly opened somewhere else entirely. Once the game settled down we were able to finalize our path and remove all of the world portals. There’s only one impossible space left in the whole game. See if you can figure out where it is.
After the map layout was finalized, most of these were fixed into boring linear spaces, likely for the sake of performance, save for one spot. Where is it?
In the entity list, there is a linked_portal_door for this purpose; it is used in the Death Trap (in map
This entity is designed to link two separate, distant areas together without any clear transition between the two. This entity is exclusive to Portal 2.
It is used in Chapter 9, in the Chamber 75 Death Trap with all the crap turrets. The room is actually completely separate from its supposed surroundings. It is the only use of this entity in the final version of the game, though commentary states that it was used a lot during the development of the title to link chambers together.
I guess that the development team first made the rooms separate so that they could wipe a bad room, then connect them later when the levels were more perfect, and I suppose that this room didn’t fit.
Here is a picture of the Death Trap that isn’t connected to the other rooms:
Near the threshold and a tiny step (noclip fly) to either side:
You might want to skip the commentary and long walk and go to 04:30. From here on he shows the box from outside at the three locations (before box, in box, after box); he first does this slowly, then at a later point he goes between all of them in quicker succession. This might show the trick better than above pictures.
Thanks to Jon Ericson in the comments for finding this video.