Quaternary fluvial deposits of the river Main overlying Pliocene, lignite bearing sequences and Miocene sands and marls form the subsurface of the town.

Sometime about AD 100, during the reign of Roman Emperor Trajan, a cohort castrum was built on what is now Seligenstadt's marketplace and parts of its old town.

The 500 legionaries and auxiliary forces stationed there belonged to the Legio XXII Primigenia (or Roman 22nd Legion), based in Mogontiacum (Mainz).

The cohort was known by the name Cohors I Civium Romanorum equitata and was responsible for security along the stretch of the Limes Germanicus running along the Main.

With the fall of the Limes as a result of raids by the Alamanni in about AD 260, the castrum was abandoned, and the Romans withdrew farther behind the Rhine line.

On the former castrum's rubble and on what is now the monastery area in a section of the Breitenbach valley arose the early mediaeval settlement of Mulinheim superior, or Obermühlheim.

[3]: 14 In 1028, a Roman Catholic synod was held, whose most important result was the introduction of ember days with their strict rules for fasting.

A royal court (or Kaiserpfalz) was built on the banks of the Main during the reign of the Staufer family, however it is not certain whether this occurred under Barbarossa or one of his successors, possibly Friedrich II.

[3]: 14 In 1527, Archbishop Albert of Mainz brought in a new town order whereby the Seligenstadt townsmen's rights were sharply limited.

During the Thirty Years' War, a Swedish commissary administered the abbey on King Gustav II Adolf's behalf.

Through the secularization of Electoral Mainz in 1803, the Amt of Seligenstadt passed to the Grand Duchy of Hesse-Darmstadt and the abbey was dissolved.

Today's structure is mostly Baroque, dating to a reconstruction on occasion of its 900th anniversary, but the nave of the original three-aisled church built by Einhard is still extant.

Although the building was heavily modified over the centuries, this is nonetheless one of the largest basilicas with a basic Carolingean structure north of the Alps.

From around the same time comes the so-called Romanisches Haus built in massive stone with great arcades on the ground floor.

The town hall at the marketplace was renovated in 1823 and stands out architecturally as the only Neoclassical building with great arcades in amongst many timber-frame houses.

The timber-frame neighbourhood along Rosengasse is called Klaa-Frankreich (Frankreich means “France” in German), for which there is a particular historical reason: After the Thirty Years' War, Abbot Leonhard Colchon settled people from a Wallonian homeland here after the local population had been decimated by warfare, famine and the Plague.

Names like Beike, Massoth, Bonifer, Dutine, Oger and Assian still bear witness to the earlier francophone settlers.

[3]: 14 The Seligenstädter Geleit (“Seligenstadt Escort”) is a custom that is unique in Germany from the Early Middle Ages, which has been preserved in altered form down to the present day.

Anyone who could not pass the so-called Nagelprobe (“nail test”) had to “treat” the merchants’ guild, and this specifically meant paying for the catering.

This custom is, in moderated form, still today in Seligenstadt the highlight of the Geleitsfest (“Escort Festival”), which is held every four years.

Nowadays the parade has more than a hundred elaborately built attractions, drawing an average of forty thousand interested visitors from near and far.

Before this there had been two or three (the exact number cannot be confirmed) ferries linking the Hessian town of Seligenstadt with the Bavarian side of the Main – the communities of Kahl and Dettingen.

It is run by the Seligenstadt Town Works and incurs great losses every year due to, among other things, the high administrative costs.