Starting with the huge Kalemegdan Fortress in Belgrade. This is the medieval Zindan gate, but most of the fortress is of the later Vauban style.
Travelling south, there is the fortified monastery of Manasija, nestled in the Resava Valley.
Then the massive Ottoman fortress at Nis. One and half miles of walls.
And finally, possibly the best of them all, Maglic in Western Serbia, towering over the Ibar Valley.
Many more photos and descriptions on the Serbia page at Balkan Military History. As well as the Serbian castles on the Danube covered on an earlier trip.