Between Dürnstein and Mautern, a small gully called the Windstallgraben cuts down toward the Danube, and somewhere along its edge stood a Roman watchtower. The Wachau valley was once the frontier of the empire, and the Danube was the line. The Romans built a chain of forts and watchtowers along the south bank, and Mautern, ancient Favianis, was one of the strongest. The Danube Limes in Austria and Germany is now part of a UNESCO World Heritage listing, and these modest remains are part of that long, thin story.
What you will actually see depends on the spot. Little of a watchtower survives above ground, so you are mainly standing in a landscape that the soldiers knew: a river bend, terraced vineyards, rocky outcrops and the dark woods that crowd the slopes. The Wachau itself is a UNESCO site as well, famous for its apricots, Grüner Veltliner and Riesling, and for villages like Dürnstein, where the English king Richard the Lionheart was held prisoner in 1192 and 1193 on his way home from the crusade.
The riding is a pleasure. The Danube road along the north bank, with the river on one side and vineyard terraces on the other, offers gentle bends, a few small climbs, and views that make you slow down willingly. In apricot blossom time in spring and during the autumn wine harvest, the valley is at its best, but both periods bring cyclists and tour coaches, so expect to travel at a sociable pace. The small ferries at Spitz and Weißenkirchen offer a charming change of banks.
For fuel, go to Krems or Mautern, where the stations are easy to find; the villages of the valley are tiny, and Dürnstein itself has barely three hundred residents. Food is a delight: look for an Heuriger, a seasonal wine tavern marked by a hung branch, where you can eat cold cuts and bread with a glass of wine, though the rider's version is a glass of apple juice spritzer. Come early on weekdays to beat the crowds.