The Danube does something unexpected just upstream of Kelheim. Instead of wandering across flat Bavarian farmland, it cuts straight through a band of white Jurassic limestone, squeezed between cliffs that rise steeply from the water. This is the Weltenburg Narrows, and on the hill above the abbey at Weltenburg, at the spot called Am Galget, the Romans put a small fort to watch the river and the frontier.
The fort was part of the Raetian frontier, the line of the Roman Limes where the Danube itself served as the border. The remains today are mostly earthworks and ground traces rather than standing walls, so this is a place you visit for its position and story more than for masonry. Stand up here and it becomes obvious why soldiers were posted: the river, the gorge and the land beyond all lie open. A few minutes away, down at the water, Weltenburg Abbey has been a Benedictine house for well over a thousand years, its church a riot of late Baroque by the Asam brothers, and its brewery is reckoned one of the oldest monastic breweries in the world. The dark beer in the beer garden is the local sacrament.
The classic way to see the gorge is by boat from Kelheim, which takes you past the rock formations that locals have given names over the centuries. On the bike, the road to Weltenburg is a dead end of sorts, so the ride is about getting there and back through the wooded hills of the Altmühl and Danube country. The roads around Kelheim, Essing and Riedenburg are gentle and well surfaced, with river views and plenty of bends through beech woods. Summer weekends bring crowds of day-trippers, cyclists and coaches, so a weekday or an early start makes a real difference.
Kelheim, about five kilometres away, has fuel, shops and everything else you might need. Above the town on the Michelsberg stands the Befreiungshalle, a huge round hall built for King Ludwig I to commemorate the wars against Napoleon, and it is worth the short detour for the view alone. In late autumn the woods turn gold and the river mist lingers well into the morning, which makes for beautiful but damp riding.