Visit Golubac Fortress – a medieval fortified town on the south side of the Danube River – before cruising through the Iron Gates. The Iron Gates is one of Europe's most awe-inspiring natural wonders. The Danube narrows as it winds through a series of magnificent gorges between the Carpathian and Balkan Mountains, forming a natural border between Serbia and Romania.  The ancient King Decebalus, the last king of Dacia, is proudly carved into the mountainside and always captivates travelers.