Over the centuries, the Danube has been one of the most significant cultural and historic boundaries in Europe. In the early Middle Ages (3rd Century A.D.), the full length of the river formed the northern border of the Roman Empire. It remained intact as late as 454 A.D. when the Goths, Huns, Slavs, and other groups crossed the Danube in order to invade the crumbling Empire.

The Roman legacy established the importance of the river as a medieval trade route, explaining why so many crucial trade and transportation centers can still be found along its shore today. The waters of the Danube also form a critical militaristic and spiritual lifeline. It was the major connection between Europe and the East, providing a pathway for crusaders to charge into Byzantium and the Holy Land.

The trade corridor along the Danube gave rise to two major empires, the Austrian and Hungarian, which merged under Austria in the early 19th century. The Danube then served as a link between the industrial centers of Germany (Austrian Empire) and the agricultural areas of the Balkan Peninsula (Ottoman Empire). It also served as a critical cultural border. To this day, Romania (north of the river) and Bulgaria (south) reflect their respective and separate histories with Romania having a Romance language and Bulgaria demonstrating key historic affinities in architecture and religion with the Ottoman Turks.

Both the commercial and military value of the Danube are still recognized today. Many treaties have been signed to try to keep one country from having too much control over the river. Today the Danube is still a major transportation route, with more than 3,500 ships passing through its delta each year. Extensive navigation is made possible by various dredging of canals and channels often constructed with inter-country cooperation for the benefit of all the nations that border this essential waterway.

As the Danube leaves the cover of German''s Black Forest, it trolls through the pristinely-preserved medieval town of Regensburg. This bustling university town boasts ancient treasures like the Porta Praetoria—the north gate to the ancient Roman fortress, Castra Regina, built in 179 A.D. During the holidays, Regensburg shines bright with its Christmas market lighting up the historic market square.

Farther downstream, Passau, Germany, is a gem of a city located at the confluence of the rivers Inn, Ilz, and Danube. Cobblestone streets lead the way to St. Stephen's Cathedral, which houses the world's largest church organ with more than 17,000 pipes.

Continuing along, the village of Grein offers excellent photo opportunities, with its turreted houses lining the main square in the shadow of its idyllic castle perched high on a bluff. Situated in the rolling green hills of the Wachau Valley, Melk is home to the magnificent Benedictine Abbey, one of Europe's largest and most intriguing monasteries. Step out onto the monastic terrace to look out over the Danube and take in the mesmerizing view of the sweeping countryside.

Float through the Wachau wine region to Dürnstein, known for the magnificent baroque Stiftskirche with its blue façade. Above town are the ruins of the castle where England's King Richard the Lionheart was imprisoned in 1192.

Vienna, Austria's capital, is next. The "City of Music" inspired the creative genius of Mozart, Beethoven and Strauss. Once the center of the mighty Habsburg Empire, the city has much to offer—the lavish Hofburg Palace, the impressive Vienna Opera House, and the majestic Ring Boulevard, to name just a few. Visitors can waltz their way down shop-lined streets, savor the sounds of an orchestral concert, or delight in a Sachertorte or Apfelstrudel in a neighborhood café.

Sail on through the picturesque Austrian views before heading to the heart of Hungary. Spanning both banks of the Danube, Budapest is Eastern Europe's liveliest and most cosmopolitan metropolis. Seven bridges, including the famous Chain and Elisabeth Bridges, connect ancient Buda on the right bank with Pest on the left. The massive hilltop castle complex with Fisherman's Bastion and the Matthias Church is among the many sights of Budapest that impress and excite.

South of Budapest, the quiet town of Kalocsa, Hungary, was founded by St. Stephen in 1009. The Archbishop's Palace and the Kalocsa Cathedral are must-see sights in this town that is most known for paprika and folklore art.

The Danube winds away from Hungary and through the city of Novi Sad, Serbia. Steeped in history, the hilltop Petrovaradin Fort affords dramatic views of the Danube. On to Serbia's capital, Belgrade, situated at the confluence of the Danube and Sava Rivers. Full of lively pedestrian areas and imposing cathedrals and fortresses, Belgrade is a riveting study in contrasts.

The Danube flows away from Belgrade, through rustic hills lined with stunning Roman fortresses, and through the dramatic gorge of the Iron Gates. This narrow and formerly very dangerous passage divides Europe's Alps in the northwest from the Carpathians in the southeast and forms a natural border between Serbia and Romania.

Downstream, Bulgaria's most ancient and breathtaking treasures can be found a short drive away from the Danube. Veliko Târnovo, situated on three hills surrounding the Yantra River, is a natural fortress with stone houses clinging to its steep slopes. The historic village of Arbanassi is nearby, featuring the well-known Etnographic Museum and intricately designed frescoes in its Church of the Nativity.

Silestra is a key port city—essential to Bulgaria's thriving agricultural industry. A short distance away, the resort town of Varna sits on the shores of the Black Sea. Its storied past comes to life as you walk through Roman baths and marvel at the Gold of Varna, an archaeological wonder.

Bucharest, Romania's lively capital, is only a short drive from the shores of the Danube. Its wide boulevards and Arc de Triomphe have gained it the name "Paris of the East." Enjoy people-watching in the Royal Palace Square, the scene of riots in 1989 which led to the collapse of the communist dictatorship. The Royal Palace now houses the National Art Museum and is filled with priceless examples of Eastern European art.

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