In 1173, Henry the Lion commissioned the building of the St. Blasii collegiate church as a Romanesque arched basilica. Henry was buried there in 1195 next to his wife Mathilde. The tomb of Kaiser Otto IV, son of Henry the Lion, can also be found in the cathedral. The crypt serves as a place of rest for Guelph princes. The mediaeval fittings are of great significance: the Imervard cross, a seven-branched candelabrum, the Maria altar, the tombs of Henry the Lion and Mathilde as well as many artistic wall paintings.