You get what you pay for at this place. It's cheaper than most Airbnbs in the area and this is why:
The building's interior is quite unappealing and the walls are paper thin.
At times you'll find people smoking in or near the hallway and the entrance.
Twice I encountered people waiting for someone to open the door so they can get in.
There is huge construction site immediately next to the apartment, with scaffolding extending to the edges of the terrace.
It can be much cleaner, and the couch pillows and blanket can use some serious washing so they don't smell like hundreds of people at once.
Positives:
The terrace. You won't find a cheap place with a terrace in Madrid.
The light. It's very bright.
The bed seemed very comfy.
There's an elevator.
The location: It's right next to Retiro, which is the central park of Madrid. It's fairly close to museums but not to the old town and the center.
I stayed a friend's place because I didn't feel comfortable and safe there, and the management company didn't do much to resolve the issue.
Overall, if you're looking for a cheap place and don't expect much, it's a great option.