We had a wonderful stay at Sultanahmet Gardens. The location cannot be beat, just down the hill from the most important historical sites in Istanbul, but also within a residential neighborhood--one of the last remaining in this region of the city. I recommend exploring the small streets around the flat, where people are friendly but do not hassle you the way they do on the more touristy streets.
Tansel's mother met us and was very thoughtful and kind. She showed us around the flat, gave advice and maps and a transportation card to use, and left breakfast food for us in the fridge. We did not get to meet Tansel, but he was very friendly and responsive when we had a question or a problem.
Be aware that the flat has no living room area, and the kitchen was pretty basic--although it had everything we needed, including a teapot and a Turkish coffee pot! We did not spend very much time in the flat because we were so busy seeing the sights of the city. However, we would have liked to spend more time in the beautiful garden if the weather had been a bit cooler.
Tansel helped us arrange transportation from the airport and was flexible and helpful in every way.