Places you should visit on your cultural route in Serbia are:  Sremska Mitrovica , a town standing on the site of Sirmium, the capital of the Roman Empire at one time, Belgrade town which have been the target of many invaders throughout its history. The best preserved of the latter in Belgrade  is Kalemegdan, the old core of the Belgrade fortress,. Visit also Lepenski vir, a unique prehistoric discovery, was brought to light about twenty years ago. The most important finds in the well laid-out settlement are a number of monumental stone sculptures made some six to seven thousand years ago.
And further on to Kladovo you will see the monuments  of eternal human desire to rule this beauty and wilderness from Roman times till nowdays: Trojan's board which dates back from 101 AD, Trojan's bridge, Diana-the biggest Roman fortress from the II nd century, then fortress Fetislam...

South of the Danube, near Zajecar, are the ruins of the late-Roman fortress Gamzigrad (3rd and 4th century). Archeological diggings in the fortress have unearthed the remains of a palace with exceptionally fine mosaics, public baths and impressive city gates.
The ancient Roman town Nais, present-day Nis, was an important frontier fortress of the Roman Empire.
The remains of the Roman summer resort Mediana, with the foundations of villas, mosaics and marble figures, have been unearthed in the vicinity of Nis. A 6th century Byzantine tomb, discovered in the city, includes well-preserved frescoes with zoomorphic and floral motifs.