Great forts are rare on the Limes; most of the frontier was watchtowers, small timber and later stone posts within sight of each other, each with a handful of men and a fire for signalling. The towers numbered 5/15 and 5/16 stood near Großkrotzenburg, at the point where the land frontier coming down from the Wetterau met the Main, and from here southward the river itself became the border as far as Miltenberg. Little of them remains to see, but the location tells the story: the Romans used water where they could and built palisades only where they had to.
Großkrotzenburg sits on top of a Roman fort, and traces of it have been found beneath the old village. Today the skyline is dominated by the chimneys and cooling tower of the big power station on the river, a slightly jarring neighbour for a heritage site, but a good landmark when you are trying to find your way across the flat country between Hanau and Aschaffenburg.
This is not mountain riding. The roads along the Main are straight, built-up and often slow, linking commuter towns where Hesse and Bavaria meet. The pleasure lies in the stops. Seligenstadt, a few kilometres upstream, is the one not to miss: a handsome old town of half-timbered houses around a market square, and a great basilica founded by Einhard, Charlemagne's biographer and adviser, who is buried there. A small car and passenger ferry crosses the Main from Seligenstadt to the Bavarian bank, and taking it on a bike is a quietly happy few minutes.
If you want curves, they are close. North of the river the hills of the Spessart rise behind Alzenau and Kahl, with forest roads that climb and twist through oak and beech, and to the south the Odenwald begins within half an hour. The Main valley itself is better treated as the link between them.
Fuel is easy: Großkrotzenburg, Kahl and Seligenstadt all have stations, and Seligenstadt has plenty of cafés around the square and along the riverbank, where in summer half the region seems to sit with an ice cream. Expect heavy traffic around Hanau at rush hour and come instead on a weekend morning, when the river is misty and the only sound is a barge working upstream.