Kalemegdan is not only one of the most beautiful outdoor areas in Belgrade, but also one of the most mysterious ones. The entire park lies on top of underground tunnels, built by Romans centuries ago. You can actually visit these tunnels and even have a glass of wine, with expert tour guides. Visit to the underground sites portray the story about the unusual history of Belgrade, strange fates and legends, as well as answers to some of the enigmas that for centuries encourage curiosity about history and culture. The tour includes visit to the Roman theater where are the foundations of the main gate of the Roman fortress from the 2nd century, Roman aqueduct and pipes. Visitors will enter in the Roman well built in the early 18th century, the underground bunkers and a large warehouse Barutana. We will pass through the tunnel that leads from the collar to the Kosancicev wreath in Karađorđeva Street, from which one can see another lagum.
see location here : link