Deniz Houses apartment is a GREAT place to stay in Istanbul and I insist on it.
Location is ideal if you want to do sightseeings, Hagia Sophia mosque, Blue mosque and grand Bazaar are very near to the hotel. Plus, the room are quiet, very clean and well equiped with slippers, tea, a bottle of water and wifi. I really recommend you stay there because it's cheap and it's worth it! The hotel staff would help you and even find you a "taksi" if needed, they are friendly.