We had a great stay at Marta's. For 3 people, there was a sofa bed, and 2 single beds (pushed together).
Marta was prompt with replies and gave great recommendations for food/coffee in the area.
Check in was seamless - the key was kept in a locker 5 minutes away and you will collect it with the instructions / code given.
The elevator leads up to the 3rd floor only, the apartment is on the 4th floor, so do note the slight walk-up! I have shared this feedback with Marta and believe that it's mentioned in the Spanish description but not specified in the English description at the time of booking. Something to consider if you need easier access!
Location is prime, just 5 mins from Plaza Mayor, and also if you are in Madrid on Sunday, right beside the El Rastro flea market! The place is not noisy as well as the listing mentions.
Overall a great stay!