This flat is everything you need for a basic sightseeing-trip to Madrid.

First, it is located in the very center of town and easily accommodates 4 people. The flat is arranged rather basic, but stylish. Located directly under the roof (you will have to climb up to the 4th floor daily) it has its downsides with standing upright in some areas or bumping your head when standing up from the toilet or table.
The beds aren't exceptionally comfortable but absolutely fine for a few days. You'll however have to be comfortable with your travel-companions if you go for the double bed as it is rather small and just has 1 duvet.
2 ACs are cooling the rather big apartment down very effectively, which is really important if the outside temperature ranges around 35°. The kitchen also proved to be fine, although the dishes are limited. It offers everything apart from a dishwasher, a mixer and an oven.
As far as communication with the host is concerned, we didn't have any problems. Only the Check-In process was a small challenge, because it was just written in Spanish. After a bit of guessing and 2 calls we did manage to get inside and were shown around by a women.
No apartment is 100% perfect. It will however be hard to find something better value-for-money in Madrid. I would recommend my friends and family to book Carol and Ivan's apartment.