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. So great grace had Riza found in the sight of Our Lord, that like His disciple of old on the sea of Genesareth, she walked over the Rhine dry-footed to the holy sacrament in St. Castor's. One day, the sacred legend goes on to say, the stream was agitated by a storm. For the first time doubt entered the maiden's heart as her foot touched the waves. Prudently tearing a prop from a neighbouring vineyard, she took it with her for a staff over the troubled waters. But after a few timid steps, she sank like St. Peter on the Galilean lake. In this wretched plight she became full of remorse for her want of faith in God. She flung the stick far away, and lifting her arms towards heaven, committed herself to the sole protection of the Almighty. At once she rose up from the waves, and arrived, with dry feet as heretofore, on the other side. More than ever after this did Riza, this saintly daughter of a saintly king, strive to excel in those works which are pleasing to God. She died within the cloister, and her bones were laid in the Castor church, near the burial-place of the saint. Soon the popular imagination canonized Riza, and her marble tomb is still to be seen in the North transept of the Castor church at Coblenz.