My stay at Marmara Guesthouse was a dream! Located in a cute little side street of the Fatih neighborhood, it's within 10-20 minutes' walk of major sights like the Aya Sofia, Sultanahmet, and Sulaymaniye Mosque.
The room was very cozy, clean, and the (free!) breakfast on the rooftop terrace was absolutely delicious! I especially appreciated the vegan and vegetarian options included in the buffet offerings. Even on a rainy/overcast day, the (enclosed) terrace was a lovely place to while away an hour with coffee and views of the Bosphorus.
The check-in process was quite smooth, and the staff were all friendly and professional; the only downside, as is indicated in the Airbnb advert, is that there is no elevator in the guesthouse. I'd recommend packing light if you plan to stay here, as the stairs to the rooms are quite winding and narrow.
I can't wait to go back to Istanbul one day, and when I do, I will certainly be staying at Marmara again.