The Franks seized Trier from Roman administration in 459. In 870, it became part of Eastern Francia, which developed into the Holy Roman Empire. Relics of Saint Matthias brought to the city initiated widespread pilgrimages. The bishops of the city grew increasingly powerful and the Archbishopric of Trier was recognized as an electorate of the empire, one of the most powerful states of Germany. The University of Trier was founded in the city in 1473. In the 17th century, the Archbishops and Prince-Electors of Trier relocated their residences to Philippsburg Castle in Ehrenbreitstein, near Koblenz. A session of the Reichstag was held in Trier in 1512, during which the demarcation of the Imperial Circles was definitively established.