The apartment is small but virtually as it is represented in the listing so no surprises there. It is very clean, and I saw someone else post about ants but I didn't see any in the apartment when I was there. It is in a great location- there's a grocery store (Dia is 2 blocks away) nearby, as well as a police station (which as a solo female traveler made me feel more safe) and tons of restaurants. You are about 1 block from the very busy Calle Gran Via. Also, I'm not sure if they put this in the listing but you need a code to get into the building, which I liked.
Something they couldn't help is after about a week of being there someone in a nearby room or floor started doing construction. It would start at times around 8 or 8:30 in the morning, and sometimes not end until after 6 at night. It was a constant pounding noise, as if someone was beating the wall or ceiling close to me with a hammer. For many this wouldn't be an issue as most people I assume come to Madrid to visit the sites and so won't be in their apartment. However, I am a consultant working for myself and I am dependent on good wifi and a quiet workplace during the day. There were several times where I had to go find a cafe to work from to do a Zoom meeting. I contacted the host about this and sent her a recording of the noise, and she was unaware of the construction. She told me to ask the maintenance man, which I did but he didn't know how long it would be going on either. I felt helpless. Some days it would stop, some days it would go all day. Again, not something the host can help but something to be aware of if you'll be working from the apartment.