There is not much to see at Großprüfening, and that is worth knowing before you arrive expecting walls. The Roman fort and its civil settlement lie on the western edge of Regensburg, close to where the Naab flows into the Danube, and what survives is mostly under fields and houses, known from excavation and aerial photographs rather than from standing masonry. The site is part of the Danube Limes, the river frontier that UNESCO listed in 2021, and its value is in the position: a garrison watching a river junction, a natural route north into what the Romans considered barbarian country.
The real Roman prize is a few kilometres downstream. Castra Regina, the legionary fortress founded under Marcus Aurelius in 179 for the Third Italian Legion, became the core of Regensburg, and pieces of its wall and the Porta Praetoria still stand in the old town, built into later houses as if nobody ever thought twice about it. The medieval city that grew on top is one of the best preserved in Germany, with the cathedral, the Stone Bridge over the Danube and the old sausage kitchen beside it that has been grilling for centuries.
Nearby, the former Benedictine abbey of Prüfening has a Romanesque church with remarkable twelfth-century wall paintings, which makes a calmer stop than the crowded centre. Sinzing, across the river, is a commuter village with a petrol station and a bakery or two, which is really all you need at the start or end of a day.
The riding around here is better than the flat Danube plain suggests. Head up the Naab valley and the road follows the river through wooded limestone country, with cliffs above the water and small villages every few kilometres. Westward, the Altmühl and the Danube gorge at Weltenburg are within easy reach, and the roads through the Jura there are twisty, scenic and busy with motorcycles on sunny Sundays. Eastward the Danube opens out into the Gäuboden, Bavaria's grain basket, where the land is flat and the riding becomes transport rather than pleasure.
Regensburg itself has fuel, food and every kind of rest you could want. Leave the bike outside the pedestrian centre and walk; the old town is compact, and an hour among its towers and lanes is a fine reward for a morning in the saddle.