In the original game, the goal was not to disturb the player during a test. Imagine a room with too many details. As you entered the room, you would start by looking at every little detail in the room before beginning the actual test. Also, the player must mix an important key from the test with a detail from the room.
Yet, you can't make the game full of basic geometry, your game would become boring.
That's the reason why I'm making this article to share with you the secret of minimalism in the Portal universe.
1-First test chamber
Let's use as a first example the first map from Portal. Pretty much every player knows it by heart at this point, but what did this room tell us about the game?
Take a look at the screenshot, you start in a well-lited glass jar, with a toilet, a radio, a bed, a hospital blood bag holder, papers and a big countdown. No exit, for the moment. But you can still see the outside! A simple room, with some watch rooms near the ceiling which lit the whole room. Nothing more. As soon as GLaDOS as finished her dialogue, the portal open and the first thing you see is yourself. This is one interesting point in minimalism. Simple details which add a lot.
When exiting the portal, you will have to make your way around the glass jar before exiting the room and entering the first test chamber. It could open into a big room where you could explore. However, the goal is to go to the first chamber as quickly as possible. The only details you can find are under the jar, 
where the orange light comes from. You can see pipes, ropes, and other props yet you can't really see them as the space between the floor and the jar is small and there is no much light going down here.
Afterward, you enter the first test chamber. What do you 
find? A pipe, a box, a button, a door, and another watching room lightning the full room. The only details you will find is a camera, a sign on top of the door which shows if it's open or not and dots on the floor and on the wall which indicates whatever is connected with the button.
2-18th test chamber
You would probably think that the more you go in the game, the more details there are? Well, you are partially right. Tests will become harder and harder with each test, and rooms will be more details. But remember what I said earlier. You don't have to put big props or complex geometries to make tour room more detail. I invite you to play Portal and play through all test chambers. You will notice that every test chambers have simple shapes. Every room is made of square and rectangle. You won't find any curves until your escape. The most complex geometry in the game is 45 degrees pannels. 
Now, let's take a look at the 18th test chamber and see how they add details with simple geometries and textures.
The main texture used is the metallic surface. Which is really useful to create details on the wall as it has some variations with the number and size of tiles in it. They used random variations on the wall. You can see some cubes extracted from the wall to give some depth to the wall.
Also, some of them are replaced with a hole and a light place behind to add lightning to the room. Also, random blocks were placed in a chaotic way to give details and keeping a square like geometry. And they used the same technic 
throughout the whole test chamber. Although the primary source of light is the watching room above the entrance of the room. They also used other watching room to lit other parts of the room.
They also add a broken wall on the second platform of the test. Yet as it's a big difference from other test chambers I won't be talking about it. But keep in mind 
the RatLab zone behind panels you can find in some tests chambers which don't add a lot of details on the room itself.
See this screenshot where you can find every technic they used. See those chaotic 
blocs, those changes of variations, those randomly placed lightning block...
Yet, the room is cubic. There is this change of texture on the wall on the lower platform (right of the screenshot) because it's useful. It didn't need to cover the entire bottom half of the wall. For me, it's more for a detail part. Well, I invite you to open a debate in the comment on what you think about those technics I'm talking about. I'm really interested in other points of views.
Conclusion + Sources:
I could have talked about a lot more technics, and ways of adding details to a room but I got the main point, and I can always make another, more advanced, article to discuss with more depth about minimalism in Portal.
I invite you to dig more into the subject. I can give you some help with some blog post I could find and which were given to me.
The first one is Yves Klein's monochrome blue for minimalism in paint and textures: En.wikipedia.org
Also, I give you this link about Artsy blue chromatic shop: Artsy.net I think you can find some great information about it and some example of this monochromatic paint. How you could use minimalist textures like this one and things like that. You can also take a look at Artsy if you like art: Artsy.net
Another source I can give you is this video: Youtube.com
I know it's not about minimalism, but if you make maps, it can be useful to follow. And in some way, it talks about the importance of minimalism in the source engine!
Also, I must credit Syy as I used a lot of what he said in the video, he is a great guy, please, go and support him!
The link to his video: Youtube.com
I must also source the developer commentary of the game, which was really inspiring to follow and listen and which gave me some clues about Portal map making!
Also, the Valve Developer Community: Developer.valvesoftware.com
Developer.valvesoftware.com
a great and powerful tool for modders and mappers!
Thank you very much for reading through the entire article. I hope you find some interesting facts. I invite you to open the discussion below on minimalism down below in the description.
Agur,
Ttiki
