The Aar is a small river that winds north from the Taunus towards the Lahn, through a narrow, wooded valley that has served as a route through the hills since ancient times. Where the Roman frontier crossed the valley near Adolfseck, today a district of Bad Schwalbach, the Romans built a fortlet to control the crossing, a small, square fortification with a garrison of a few dozen soldiers. The remains have been excavated and are marked on the ground, and the stretch of frontier nearby is part of the World Heritage frontiers of the Roman Empire. Above the valley, the ruins of a medieval castle, also called Adolfseck, add another layer of history.
Bad Schwalbach, two and a half kilometres away, is a spa town that became fashionable in the eighteenth and nineteenth centuries, when the iron-rich waters of its springs drew visitors from across Europe. Its spa park, with old trees, pavilions and the elegant buildings of the Kurhaus, has a faded charm. The town was formerly known as Langenschwalbach, and it has a pleasant old centre of half-timbered houses.
Downstream, the Aar valley narrows, and the ruins of Hohenstein castle stand on a rocky spur above the river, one of the most dramatic castle settings in the Taunus. In summer, open-air theatre and music performances are held in the castle courtyard. To the south, across the ridge, the Rheingau slopes down to the Rhine in a sea of vineyards, producing some of the finest rieslings in the world, with villages like Eltville, Kiedrich and Rüdesheim.
The B54, the Aartalstraße, follows the valley with flowing curves through the forest, though it can be busy with traffic heading to and from Wiesbaden. Smaller roads climb out of the valley over the ridges to Schlangenbad, another spa town, and down into the Rheingau, with views across the Rhine. Watch for cyclists, deer and speed cameras. In Bad Schwalbach you can fill up and eat well; Taunusstein, a few kilometres to the east, also has services. A glass of Rheingau riesling at a wine tavern overlooking the Rhine is the perfect end to a ride in this area.