Incredibly central, large apartment with enough beds and space for a big group (we were 9 male friends). You are out the front door and in to one of Madrid's main squares. The lift only goes from the first floor, and is a bit small and slow, but this is to be expected from an old building like this. The apartment itself is nice and clean, with one main bedroom, with three beds and a large open plan area with 4 beds in an alcove and a couple more sofa beds/pull out beds. Along with this, there are two bathrooms (one is huge with a jacuzzi bath) and a kitchen and dining room. The pull out beds are fine for a weekend, but you mightn't be too happy sleeping for longer or if you have a bad back.
Communication with Anouk and Patricia was great, and they gave us very detailed instructions on how to get to the airport to the apartment, and again instructions on what to do when leaving. All very useful and straight forward.
Overall, there is little wrong with this apartment, and I would have no issues recommending Anouk's apartment if you are visiting Madrid with a large group