In the first quarter of the 9th century, when the pious Ludwig, son of Charlemagne, was struggling with his misguided children for the imperial crown, a church was built in Coblenz to St. Castor, the missionary who had spread christianity in the valley of the Moselle. The four-towered edifice arose on a branch of the Rhine.

The palace of the Frankish king stood at this time on the highest south-western point of Coblenz, on the site of a former Roman fort, and near by was a nunnery, dedicated to St. Castor. In this building lived Riza, a daughter of Ludwig the Pious, who had early dedicated her life to the church. Every day this king's daughter went to mass in the Castor church on the opposite side of the Rhine.