Mert was a great and welcoming host! He met us right from the taxi and helped us with the bags, the same happened on our departure. He was caring about all aspects of our stay very well.
The studio is as advertised: small but has everything you'd need. First floor (second above ground), but it's not a problem. Bed, sofa and a kitchen with almost anything for cooking. Personnel will help with anything you need (cleaning, some supplies). Wifi is good. The only thing, that I think is common in Istanbul in this time of year, is that it was a bit cold sometimes, but the AC was helping to keep the temperature up so it wasn't a great problem.
The house is on a street that is inclined almost 45 degrees, so keep that in mind. 20 meters up it gets plain though: Taksim square is just meters away, but no noise is heard in the apartment, as it is 2 streets away from traffic. You can get to pleasant "hotel area" near the square or main streets of Istiklal and Sıraselviler in 3 minutes. So the location is quite central.
Overall, big thanks to Mert for making this stay so comfortable.