Fantastic stay. Small apartmentperfect for one or two travelers, but has everything you could need. The beautiful terrace offers a great view in the morning. You are on the Asian side in a very nice location with everything you could need. There is a metro and Ferry station a quick walk from the apartment and public transport in Istanbul is cheap and easy.
Mustafa was a grea host and made things easy. I would absolutely stay again.
As others have mentioned it is on the 5th floor with no elevator so if you are physically limited keep that in mind.