Seligenstadt is one of those Main valley towns where every era has left something on the table. The Romans came first: here the frontier of the Upper German-Raetian Limes ran along the river itself, and a fort with a civilian settlement around it guarded the Main crossing. The fort site lies under the old town, so you are walking over it rather than looking at it, but the logic of the place is obvious when you stand by the water. The river was the border, the road and the supply line all at once.
The town's real fame comes from a few centuries later. Einhard, the scholar and courtier who wrote the famous biography of Charlemagne, founded an abbey here in the ninth century and brought relics of the martyrs Marcellinus and Peter, and the great basilica still dominates the town. Einhard and his wife Imma are buried in it, and there is a romantic legend about Imma being Charlemagne's daughter who eloped with Einhard; nice story, poor evidence. The abbey buildings and their baroque gardens behind the church are worth an hour, and the half-timbered houses around the market square are as handsome as anything in Hesse.
Riders come for a stop more than for the roads, which are flat around here. The Main plain between Hanau and Aschaffenburg is busy, built-up and crossed by railway lines and motorways, so this is a place to pause on the way to better things. Those better things are close: south into the Odenwald, with its tight valleys and forest climbs, or east into the Spessart, whose quiet roads through deep beech woods are a favourite with bikers from the Rhine-Main area. A small car ferry crosses the Main here to the Bavarian bank, a pleasant two-minute ride when it runs.
Practically, Seligenstadt has everything: fuel at the edge of town, plenty of ice cream, cafés and beer gardens in summer, and benches by the river where you can watch barges push slowly upstream. The old town streets are cobbled and narrow, some pedestrianised, so park outside the centre and walk in. Summer weekends are busy with day-trippers from Frankfurt, so a weekday or an early evening visit is calmer, with the low sun turning the basilica's sandstone a warm red.