I've been visiting Istanbul at least once per year for the past decade, and Tamer's place is my favourite of all the various hotels, Airbnbs and friend's apartments I've stayed in.
The views from the rooftop are incredible (even more impressive than they look in the photos), and the location is perfect, as it's a short walk from Taksim, an even shorter walk down to the trams and ferries in Karaköy, and just around the corner from the Şişhane metro station. Plus when it comes to restaurants, cafés, bars, art galleries, shopping, colourful street life, etc, the Beyoğlu district is the best place to be in the city.
The apartment itself is in a beautiful historical building with super high ceilings, wooden floors and loads of character. And Tamer is a great host and cool guy, who makes sure to take care of all the small details so guests really feel at home. I'd definitely stay here again on my next trip, and hopefully for a longer period of time.