Around the year 100 AD, during the reign of the Roman Emperor Trajan, a cohort fort was built in the area 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 safety of the Limes section running along the Main (also: Upper Germanic Limes). With the fall of the Limes during the Alamanni storms around 260 AD, the fort was abandoned and the Romans withdrew behind the Rhine line again. The early medieval settlement of Mulinheim superior, Obermühlheim, was built on the rubble of the former fort and on today's monastery area in the valley section of the Breitenbach.
The oldest known written mention of Seligenstadt, then called Obermühlheim, goes back to the donation of Ludwig I to Einhard and, according to a copy of the deed of donation in the Codex Laureshamensis, dated January 11, 815. The city was founded by Einhard, the biographer of Charlemagne , founded. After receiving the Franconian estate of Obermulinheim as a gift from Louis the Pious in 815, he founded a Benedictine monastery here. A Count Drogo is named as a previous owner. The bones of the martyrs Peter and Marcellinus, stolen in Rome, were transferred from the basilica in Steinbach im Odenwald to Obermühlheim in 828, which thus became a place of pilgrimage. The place name soon changed from Obermühlheim to Seligenstadt. The remains of the martyrs were initially kept in the St. Lawrence Chapel on the estate, but this proved to be too small given the influx of believers. Einhard quickly 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 its own monastery.
Source: Wikipedia