Niš Fortress in Gradska opstina Crveni Krst

You're standing at Niš Fortress, known in Serbian as Niška tvrđava. It sits on the right bank of the Nišava, the river that runs through the city. Take a good look at those walls. On average they're eight metres high and three metres thick, and together they run for more than two kilometres. Inside lies around twenty-two hectares of land. So this isn't some little castle. It's an entire walled district.