Around the year 100 AD, during the reign of the Roman Emperor Trajan, a cohort fort was built on the site of today's Seligenstadt market square and parts of today's old town, the Castrum Selgum. The cohort stationed there was called Cohors I Civium Romanorum equitata and was responsible for the security of the Limes section along the Main (also known as the Upper Germanic Limes). When the Limes fell during the Alemanni storms around the year 260 AD, the fort was abandoned and the Romans retreated back behind the Rhine line. The early medieval settlement of Mulinheim superior, Obermühlheim, was built on the ruins of the former fort and on the current monastery grounds in the valley section of the Breitenbach.
The oldest known written mention of Seligenstadt, then known as Obermühlheim, goes back to the donation from Ludwig I to Einhard and, according to a copy of the donation document in the Codex Laureshamensis, dated January 11, 815. The town was founded by Einhard, the biographer of Charlemagne. After he received the Franconian domain of Obermulinheim as a donation from Ludwig the Pious in 815, he founded a Benedictine monastery here. A Count Drogo is mentioned as a previous owner. The bones of the martyrs Peter and Marcellinus, stolen in Rome, were transferred from the basilica in Steinbach in the Odenwald to Obermühlheim in 828, which thus became a place of pilgrimage. The name of the town soon changed from Obermühlheim to Seligenstadt. The bones of the martyrs were initially kept in the Laurentius Chapel on the estate, but this proved to be too small given the influx of believers. Einhard therefore promptly began building the Einhard Basilica, the landmark of the city on the Lower Main, and, as its first lay abbot, founded a Benedictine abbey as his own monastery.
Source: Wikipedia