When you visit Belgrade, Serbia you will surely not miss to visit the Belgrade Fortress. It is really huge and has a lot of history. Plan to spend several hours (if not days) there. Over time the fortress has changed a lot and often. The website has a lot of information (see below) and if you decide to hire a guide, you will get much more information.
"The Belgrade Fortress was built on the ridge above the confluence of the Sava and the Danube in the period from the 1st to the 18th century as a complex of a defensive character. Today, the fortress is a kind of museum of the history of Belgrade. High above the Sava and Danube confluence, on the rocky ridge which opens the view of Novi Beograd, Zemun and wide plains of Pannonia, there is the Belgrade Fortress with Kalemegdan, the former historical and urban center of Belgrade. This spatial complex consists of: The Fortress, divided into Upper Town and Lower Town, and the Kalemegdan park, the most popular promenade for Belgrade citizens.
Because of its exceptional strategic significance, at the end of the I century A.D. the Romans built a fortress here - Roman castrum, as a permanent military camp of the IV Flavius' legion. After its destruction by the Goths and Huns, it was reconstructed in the first decades of the VI century. Less than a hundred years later, it was destroyed by the Avars and Slavs.
Along the fortress on the hill above the Sava and Danube confluence, the ancient Singidunum has been raised up, and, at the same place the Slavs made their Beograd. The mediaeval town has developed for centuries in the Fortress area, searching for safety within its thick walls. The Belgrade Fortress has been destroyed and renewed for many times. Above the Roman walls there are the Serbian, and above these the Turkish and Austrian fortifications. In the XII century, the Byzantine Czar Manuel Comnenus erected a new castle upon the Roman ruins. In the first decades of the XIV century, this small stronghold on the hill was expanded to the river banks.
As a new center of Serbia, under the rule of Despot Stefan Lazarevic, Belgrade has been fortified with wide forts of Upper and Lower Towns. At the old castle, despot's palace has been built, and a war port has been built on the Sava. Within the walls, a prosperous mediaeval town has been developed. From 1521, when it was conquered by the Turks, until the XVII century, the Belgrade Fortress has not been significantly expanded.
A new period began with the Austrian-Turkish war. Having been the key fortress in the center of war actions during the XVIII century, it has been reconstructed three times. The old castle has been demolished, and a large part of mediaeval walls was covered by new fortifications. Under the Austrian rule, from 1717 to 1739, after the construction of new, modern forts, the Belgrade Fortress became one of the strongest military fortifications in Europe. It was built by the design of the general Nikola Doksat Demorez, the Swiss in Austrian service. His destiny was to be shot right in front of the walls of the Fortress he designed, in a March morning in 1738, because he had lost the Niš battle. All of the new-built fortifications have been destroyed before the Turks returned to Belgrade in 1740. By the end of the XVIII century the Belgrade Fortress got its final shape. In the war destructions during the past decades almost all structures in Upper and Lower Towns have been demolished, while the wall have been considerably damaged.
The Knez Mihailova and Uzun Mirkova streets lead to the Belgrade Fortress. At that direction are the main Fortress gates - Stambol Gate (inner and outer) and Sahat Gate. The entrance to the mediaeval fortress was on the eastern side (near today's Zoo), through Zindan Gate and Despot's Gate of Upper Town. The access to Lower Town is from Bulevar Vojvode Bojovica (Vidin Gate) and from Karadordeva Street (Dark Gate)."
Source: www.beograd.rs/en/sights/201323-belgrade-fortress/