I will try to be as fair as possible in my review, and therefore divide the accommodation description into two parts: positive and negative points.
Positives - The ad is very realistic about what is offered. It is a small room with a mezzanine . Inside the room there is a table, a chair, some wooden stairs and a bed on the mezzanine (or rather, a mattress on a platform). It's not a luxury room but it's what we pay for, and to spend two or three nights is enough. The rest of the house has a small living room with a sofa and TV (which is adjacent to the bedroom, there may be a problem with noise), a kitchen and a bathroom, as well as access to a small patio. There are two additional bedrooms for other visitors. The location of the accommodation is excellent, very central, with a metro station on the doorstep and within walking distance of most tourist sites in the city. Johan is trivial and easy to communicate before and during check-in, giving tips and information to visit the city. Check-out is independent, which also facilitates the process. So far, nothing negative to point out.
Negative points - The problems start when we start exploring the house a little more and we realize the state of hygiene of everything. I'm going to use the word disgusting because that's what I felt. Specific examples: The sheets that were placed on our bed were extremely old and worn and were full of unknown stains and hair. I even dare say that they were not changed after the previous visitors had left, such was the state of dirt, hair and shabby as they were. The hair was also scattered around the room. The sofa in the living room equally dirty and stained, we didn't even dare to use. In the kitchen the dirt continues, a dirty electric stove, a completely grimy microwave that is disgusting to use. The bathroom continues, hair in abundance on the floor, on the toilet, on the shower floor. There was toilet paper from previous visitors inside the waste bin. There was a pad at the front of the shower to place your feet that looked like it hadn't been changed in a long time, and contrary to what I saw described by the owner in some comments, there was no any cleaning of the common areas during the days we were there... anyway, I don't want to continue, and I'm sorry to have to write this, but in fact the lack of hygiene created a very negative impression on us, and it's a shame considering the potential of the apartment and its location. It is an advice that I give to Johan, bet on a thorough cleaning between visitors to avoid these situations.
I would like to point out just a few more issues to bear in mind: the bedroom's small window overlooks the patio, but it has no blind or way of blocking the light, this can be problematic for those who are sensitive to light. On the other hand, there is a continuous noise and vibrations that I think might be from the subway, in the distance. When you're trying to fall asleep it can be irritating too.
I end the review by repeating that the apartment is a great option for visiting the city, at a fair and great cost to explore madrid. But in fact the issue of general hygiene ended up spoiling the experience a little.