Up in the hills above the Altmühl valley, west of Neustadt an der Donau and the Danube, the old Raetian frontier crossed some of its prettiest country. The numbers 15/27 to 15/47 cover a long stretch of the line, and because that line follows the ridges and plateau edges, the landscape is a mixture of fields, juniper-dotted limestone slopes and sheltered woods. The Altmühl itself is a slow river that has cut a deep, winding valley through Jurassic limestone, and the whole district is a nature park, known for fossils from the quarries at Solnhofen.
Riedenburg, the nearest proper town, sits at the base of the cliffs and is dominated by the Rosenburg castle above it, which has a falconry display in season. Essing, a few kilometres downstream, is a tiny village under a rock face with one of Europe's longest wooden footbridges crossing the canal, a lovely piece of carpentry that appears in many a photograph. Burg Prunn, a medieval castle on a ledge above the river, once held a famous manuscript of the Nibelungenlied. Nearby Kelheim marks the point where the Altmühl joins the Danube.
The roads here are a pleasure. The route follows the river for much of the time, a gentle meander with trees and cliffs close at hand, then climbs out onto the plateau where views open across fields and woods. Corners are rolling rather than tight, and the surface is mostly smooth, apart from some patchy repairs. On sunny weekends the valley is busy with cyclists and canoeists, so the morning is best, and midweek is better. Pass with care, and remember that deer cross at dusk.
Riedenburg has fuel, a supermarket and several cafés by the river, and Neustadt an der Donau is larger and has everything. Wherever you stop, ask for the local beer, which in Bavaria is as much a part of the landscape as the limestone. The Limes itself is quiet and unmarked in many places, so go for the country first and the Romans second.