Fortresses

Forts and citadels were built to be unconquerable, but nonetheless they outlasted rulers, wars, and many people, their remains tell a story of corruptness and that they were conquerable. Travel Serbia - Fortress

There are just about 40 citadels or once existing forts. One of the most remarkable forts is the medieval fort of Golubac at the entrance to Djerdap canyon. The same canyon conceals secrets of Diana - Roman fort near Kladovo. Witnesses of the creation and the doom of the first Serbian state are Ras near Novi Pazar and Maglic on the Kraljevo - Raska road, and the witness of its definitive end is Smederevo fortress. When the land was conquered by Ottomans, they also started to build fortresses: the fortresses of Nis and Fetislam (near Kladovo) now stand as reminders of glorious times of Ottoman Empire. One of the largest fortresses in the Europe is Petrovaradin fortress.
And there is, of course, Kalemegdan fortress, around which Belgrade is built.