Kölner Bucht — the Cologne Bay — is what Germans call this region, and the name fits the geology. The Rhine slips out of the Middle Rhine Valley near Bonn and spreads into a wide, flat basin before the river presses north toward the Lower Rhine and the sea. The basin has been continuously inhabited since the Romans called Cologne Colonia Claudia Ara Agrippinensium, and the layers of that occupation surface everywhere: Roman walls embedded in modern streets, a Romanesque skyline at Bonn, a Gothic cathedral at Cologne that survived a war that flattened most of the city around it. Three million people now share this basin, making it one of Germany's most densely populated regions, but it remains remarkably green — less industrialized than the Ruhr to the north, less wild than the Eifel to the south, and threaded with parks, forests, and the river itself.

Bonn was the federal capital of West Germany from 1949 until reunification, and even after Berlin took back the role in 1999, much of the federal machinery never left. The Haus der Geschichte — the House of History — remains one of the country's great free museums. Beethoven was born here in 1770, and his birthplace is now a museum on the Bonngasse. Cologne, twenty-five kilometers downstream, is the largest city in North Rhine-Westphalia and the cathedral that anchors its skyline took six centuries to finish; its twin towers, completed in 1880, were briefly the tallest structures in the world. The two cities split the region's cultural weight between them. Their light-rail systems, the Stadtbahn networks of the Kölner Verkehrs-Betriebe and Bonn's own operator, even connect along shared lines — a daily reminder that the Lowland is one region with two centers, not two cities separated by suburbs.

South of Bonn, where the Rhine bends and the Siebengebirge — the Seven Mountains — rise from the water, sits the Drachenfels: 321 meters of basalt with a ruined castle on top. Lord Byron wrote about it. So did half the Romantic poets of Europe. According to local legend it is one of the most frequently climbed mountains in the world, served since the late nineteenth century by a cog railway that grinds up the slope. At the summit you find the original castle ruin from the twelfth century, a newer castle from the nineteenth, and a hall dedicated to the Nibelungen — to Siegfried, who in the saga slew the dragon that gave the mountain its name. From the top, the Rhine and its basin spread out below, vineyards on one side, cities on the other.

From November through Lent, the Cologne carnival owns the basin. The peak comes the week before Ash Wednesday, when crowds spill from Cologne and Bonn into the surrounding towns; Brühl, halfway between the two, hosts UNESCO-listed eighteenth-century palaces — Augustusburg and Falkenlust, built for the Wittelsbach prince-electors who ruled Cologne for almost two centuries. The amusement park Phantasialand sits nearby. Königswinter clings to the Rhine just below the Drachenfels; Siegburg, on the Sieg river, sends its ICE high-speed trains north and south as the region's eastern rail anchor. And running through it all is the Kölsch-Alt frontier, an invisible border that exists nowhere on a map but in every Rhineland pub. Some Kölsch enthusiasts, the joke goes, replace the Alt key on their keyboards with a Kölsch key. It is told with a straight face.