We stayed for 6 nights at Tulip. First you will notice it's more like small hotel, not quite guesthouse. Rooms are not big but it's more than enough for two persons. Both room and bathroom were very clean, personel is very friendly and willing to help you with any issue. Very decent breakfast is on top floor, with nice view on Bosphorus and Asian side.
Location is perfect, some 100 meters from Blue Mosque and Aya Sofya, in very quit street.
Great value for money, highly recommended.