From its source in south-west Germany, coursing through central Europe and the Balkans, the Danube is a river of romance, passing medieval castles, baroque churches and rococo palaces. Europe's second-longest river, it flows through proud cities- Vienna, Budapest, Bratislava- forming the frontier between Romania and Bulgaria before branching into a delta and pouring into the Black Sea. This Way the Danube sails along the navigable stretch of the river between Passau and Mohács, near the Hungarian border with Croatia.