Wow. First off, the location cannot be overstated. Just off the main path in old town Istanbul. You are halfway down the hill from the Hagia Sophia and Blue Mosque. A very easy five minute walk from both. Five minutes the other way is the Bosphorus Strait, which you can watch from their terrace, which can open up for a stunning view when the weather is right. Oh, and the aforementioned old town has tons of dining options, some or which are among the best rates in the city.
Next, the accomodations. Breakfast every morning is unbelievably generous and well prepared. The guesthouse also operates a tour company so they're very knowledgeable on local options.
The rooms are really comfortable...I had just left a villa that we rented for 18 people and a couple tens of thousands of dollars. These beds were way more comfortable.
Great English skills is a bonus.
Just a wonderful stay. Well done.