Koblenz is a medium size city of about 120,000 people at the confluence of the Moselle and Rhine rivers, called the Deutsches Eck or German Corner. It is one of the oldest cities in Europe, established as a Roman military post in 8 BC.
The city is small enough to see the sights on foot, which include interesting architecture, churches, museums and the huge Ehrenbreitstein Fortress across the Rhine.
The fortress is one of the largest in Europe and was built by the Prussians in the early 1800s to guard the middle Rhine region from the periodic French invasions. The fortress was never attacked, however.
Several years ago we visited Koblenz briefly as part of another river boat tour.