The history of the Belgrade fortress, the oldest urban settlement in Belgrade, dates back to prehistoric times. The city’s first recorded name – Singidunum – is of Celtic origin and the Celts were the first inhabitants of the settlement at the confluence of the Sava and Danube rivers. The Slav name of the fortress – Belgrade (Beograd in Serbian) – was mentioned for the first time in 878 in a letter written by Pope John III to Bulgarian Prince Boris. The present day appearance of the Belgrade Fortress as a fortified whole was shaped during a period of almost two millennia. The building of the ramparts, their destruction and reconstruction reflect the various stages in the rise and fall of Belgrade and its significant role in the past. Located at the crossroads of European roads towards East, for many centuries it was the defense stronghold and departure point for numerous invasions. The origins of the Belgrade Fortress and its oldest nucleus, the Upper Bourg, are related to the presence of the Romans at the turn of the eras. During the long period of their building and rebuilding, the fortifications evolved from an ancient fortified military camp, i.e. castrum, over a mediaeval fortified town to a modern bastion of artillery fortress. After the fall of the Roman Empire, in the period between the fifth and the fifteenth centuries, the Fortress had a turbulent history under the Byzantine, Bulgarian, Hungarian and Serbian rule. During the rule of despot Stefan Lazarević (1404–1427) the Fortress was a developed mediaeval fortification, with the Inner and Lower Bourg, and the western and eastern baileys. The Inner Bourg was the strongest fortification; according to despot’s biographer Konstantin Filozof, the court of despot Stefan, the treasury and the library were located there. As of 1521 the Fortress was under the Turks and began to spread beyond the walls. Modernization began with the arrival of the Austrians in 1717 and the general appearance of the Fortress underwent important changes. Powder magazine, barracks and other military buildings were built in the Lower Bourg; all Turkish buildings on the Upper Bourg plateau were demolished, except for one mosque. When the Serbs finally received the keys to the Belgrade Fortress in 1867, it remained subjected to the requirements of the Serbian, and later Yugoslav army. The Fortress played its last great martial role during World War I. From the spatial and functional aspect, the complex of the Belgrade Fortress has indivisible from the Kalemegdan Park since the 1870s when the overall transformation began. Particularly important features of the park are its public monuments, erected without interruption since the beginning of the twentieth century.