Between April and October, long and narrow vessels ply the Rhine, Main, and Danube rivers in Europe, offering visitors a different perspective on some of the cities and countryside most explore by land. Viking River Cruises is one outfitter to offer the trips, but there are others, too. The most popular of the cruises follow the Rhine from Amsterdam to Basel, passing castles, Romanesque churches, vineyards, and more. Along the way, the ships usually stop to let passengers explore in Cologne and Koblenz, home to Ehrenbreitstein, one of the largest castles in Europe. Side trips along the Moselle and Main rivers bring passengers to historic cities such as Trier (which dates back to Roman times) and Nuremberg. Cruises on the Danube continue this history lesson, traveling from the Black Forest in Germany to the Black Sea in Romania. Along the way, these cruises motor through Vienna, Budapest, and Linz, the capital of the up-and-coming Austrian wine country. Some itineraries even include stops at Bratislava and Belgrade, traversing the gorges that divide the Carpathian and Balkan mountains.