There are many cocktail bars in Istanbul and they are often located on rooftops offering amazing views of the city.
Georges Hotel, Serdar-i-Ekrem Cd. 24, Galata
Probably the nicest rooftop in Istanbul. Very secluded and private.
Mesrutiyet Cd. 67, Pera
Beautiful views of the Golden Horn. After dinner this place turns into a night club.
Marmara Pera Hotel, Mesrutiyet Cd. 15, Pera
Bar on top of the Marmara Pera hotel and part of Mikla restaurant with Danish and Turkish fusion cuisine.
Siraselviler Cd. 47, Taksim
Quality-wise, this restaurant probably offers the best cocktails in town.