The Main river is where the Roman frontier changes character. Coming down from the north, the Limes runs in a long arc through the Wetterau and then meets the river near Großkrotzenburg, and from there the Romans used the Main itself as the boundary, with towers and forts strung along its bank. The numbers 5/13 and 5/14 belong to that river line, and a fort and its civilian settlement sit close by. On the ground, expect a marker, a low rise and a view down toward the water rather than anything dramatic.
Großkrotzenburg's Roman fort was a solid one, with a garrison that controlled the river crossing, and the finds from it show soldiers from far afield. What you notice now, apart from the history, is how industrial the lower Main has become: a big power station stands on the riverbank, barges push upstream with gravel and containers, and the motorways and railway lines are never far. It is a landscape where Roman, medieval and modern Europe are laid on top of one another, and the contrast is oddly satisfying.
Ride away from the river and the country changes. The Spessart, one of the biggest continuous woodlands in Germany, begins a short distance upstream, and its oak and beech forests are famous for timber, once floated to the Dutch shipyards. The roads into the hills are smooth, with long bends, gentle gradients and little traffic on weekdays, and they make an excellent afternoon. On the plain, riding is easy but built-up, and commuters from Frankfurt fill the roads at rush hour. Spring and autumn are best, and morning mist on the river is lovely.
Hanau, five kilometres away, is the big town, famous as the birthplace of the Brothers Grimm, and it has stations, shops and plenty of cafés. Großkrotzenburg and Kahl am Main also have fuel and bakeries, so there is no need to carry spare. Local food is solid Hessian and Franconian, with schnitzel, potato dishes and apple wine. Spend a quarter of an hour at the marker, look across the river, and imagine the empire ending at the water's edge.