Diyarbakır
The old city walls of Diyarbakır. Photo via Wikimedia Commons.
Diyarbakır is ringed by 5.5 km of black basalt Roman walls, among the longest and best-preserved city walls on earth. Inside: the great Ulu Mosque, the Hasan Paşa Hanı caravanserai and the Hevsel Gardens on the Tigris, a UNESCO landscape.