 Golubac , in the Braničevo District of north @-@ eastern Serbia and on the modern @-@ day border with Romania , marks the entrance to the Đerdap national park . It is strategically located on the embankment of the Danube River where it narrows to form the Iron Gate gorge , allowing for the regulation and taxation of traffic across and along the river . In the Middle Ages , this was done with the aid of a strong chain connected to Babakaj , a rock on the far side of the river . 
