The numbers here, 13/46 to 13/49, belong to a surveyor's grid, but the line they follow is the old Raetian frontier, which the Romans built across Bavaria in the second century. East of the town of Gunzenhausen it was turned from a simple timber palisade into a proper stone wall, a rarity in the Roman north. The locals later called this wall the Teufelsmauer, the Devil's Wall, because they could not imagine human hands had built something so straight and so long. Folk stories said that the devil had built it overnight, and that he had run out of time just before dawn.
Today the wall has mostly gone, robbed for farm buildings and village churches over the centuries. A few stretches survive as low humps in the fields, grassy ridges where a plough has changed course. It takes some imagination to read them, but I find the quiet useful: you stand on a mild rise in a field of wheat, with swallows overhead and a village church tower in the distance, and the empire feels very long ago and, because of that, strangely close. The Roman army needed a long tradition of building and logistics to do this far from home, and they left behind little else except the bones of their buildings.
The riding is a lowland pleasure. Roads around Gunzenhausen cross the Altmühl valley and run among cherry orchards and low woods. The corners are long, the climbs are short, and cycle routes follow many of the same roads, so be ready to meet riders with panniers. Franconian villages are neat, with sandstone churches and a Gasthaus on the corner that serves a very decent schnitzel and a local beer brewed within a few miles. Bratwurst in a roll is the right lunch.
Gunzenhausen is only a few kilometres away, with fuel, supermarkets and a pleasant old centre of half-timbered houses along the Altmühl. Muhr am See is closer and quieter, with cafés on the shore of the Altmühlsee. Summer weekends are crowded around the lakes, so aim for a weekday or early morning.