The Diocese of Sion was founded in Octodurum, now called Martigny in the early 4th century. In 589 the bishop, St. Heliodorus, transferred the see to Sion, as Octodurum was frequently endangered by the inundations of the Rhone and the Drance. Very little is known about the early Bishops and the early churches in Sion. However, in the late 10th century the last King of Upper Burgundy Rudolph III, granted the County of Valais to Bishop Hugo (998–1017). The combination of spiritual and secular power made the Prince-Bishops the most powerful nobles in the Upper Rhone valley. Sion became the political and religious center of the region. By the 12th century they began building impressive churches and castles in Sion to represent their power and administer their estates. Valère, as the residence of the cathedral chapter in Sion, was one-third of the administrative center of the powerful Diocese of Sion. In the 12th century the Cathedral Notre Dame de Sion (du Glarier) was built in the town below Valère hill. The Cathedral became the seat of the Diocese of Sion, while the Prince-Bishop of Sion lived in the castle.