The location of the apartment is the best: the Galata Tower, İstiklal street, many little restaurants and shops are in 5 minutes walk, so is a metro station. It's also easy to get to a tram stop and a ferry pier.
The apartment itself is very charming, with a lot of unique details. The ceilings are high, beds are comfortable, kitchen is equipped with everything you can think of.
Merthan was so very kind, answered all our questions, was helping us with everything we could need, even brought us to the airport after our stay when we have a timing problem with a shuttle. He also seems to know everyone in the neighborhood, which was also very helpful - we had a place to leave the keys if we needed to split (there were four of us), and one of the neighbors even returned as a wallet that we lost downstairs!
Definitely would stay again if going to Istanbul.