Our family stayed at Eren's place for 10 days. It was a lovely vacation for us in Istanbul. The city overall is very hilly, but the location of the apartment makes it so easy to reach everything. We were able to explore both Sultanahmed and Katabas (and beyond) through the nearby tram station. Please don't limit yourself to just the tourist areas. From the apartment, you can easily get to a nearby ferry dock used by the locals, and from there you can go to the Asian side - not just Kadikoy, but also other Asian ports.
If you love to experience the local vibe, definitely stay here! During the winter time, you would almost be the only tourist in the neighborhood. It's a few blocks away from the busy shopping street, yet so quite and original in its own unique style. We highly recommend this place. Host Eren is very helpful and great with communication.