The most beautiful photos of Cagliari come from the vantage points of the Torre del Elefante and Torre San Pancrazio towers. It is not possible to take a photo of Cagliari at night from this angle because the towers are closed at this time. However, you can buy this photo in Cagliari from local photographers.
You can ask almost any inhabitant to have your picture taken in the streets of Cagliari - the locals are friendly to tourists. You don't want to take photos of Cagliari at night - you might as well kiss your camera goodbye for good.