The basilica dates back to the days of Duke Liudolf of Swabia in the 10th century and is the only church in the world dedicated to Saint Peter and Saint Alexander. In 982 Aschaffenburg – and therefore the former abbey – was incorporated into Mainz and the church and monastic college came to be a dominant factor in the Mainz archbishop's choice of residence.
https://www.info-aschaffenburg.de/en/tourism/tourist-attractions.html