From the Visit Nis tourism website:
"Nis Fortress is located in the core of the city, on the bank of the Nisava river. It is one of the best preserved and most beautiful medieval fortresses in central Balkans. It was pulled down and restored a number of times, by the Romans, over the Byzantines, Slavs, Bulgarians, and Serbs. Its assumed its final shape in 1723, when the Turks built it, facing the end of their reign in this area.
The Fortress covers 22 hectares, its walls are 2,100 meters long, 8 meters high, and, on average, 3 meters wide. Apart from the well preserved stone walls and gates, the Fortress exhibits numerous remains from the turbulent history of Nis. It is also a park, green oasis in the centre of the city , with a lot of caffes and children playgrounds."