<P> After renovations and extensions in the 11th, 12th and 14th centuries, the cathedral was completely destroyed during an air raid on 22 March 1945 and rebuilt from 1950 to 1960 . A thorough renovation of the cathedral began in 2010, including technical and conservation measures . Some of the cathedral's treasures have been shown further afield, including at an exhibition at New York's Metropolitan Museum of Art . The cathedral was reopened on 15 August 2014 . </P> <P> After the establishment of the Diocese of Hildesheim in 815 by Louis the Pious, a Chapel of St. Mary was built on the locations of the modern apse . Bishop Gunthar of Hildesheim, who was in office from 815 to 834, had a small basilica with two round towers built immediately to the south of the chapel, which he dedicated to Saint Cecilia . This served as the original cathedral and Stift church . The first four bishops were buried there . Only traces of the foundations of these two buildings remain . An older Hildesheim parish church probably once stood on the site of the Chapel of Saint Stephen next to the gatehouse at the eastern entrance to the chapel of St. Hellweg, which might date back to Hildegrim of Châlons and his expedition to East Saxony . </P> <P> The Cathedral was built in 872 under Bishop Altfrid as a cruciform three - aisled basilica with a two - story westwork . It is an example of Ottonian architecture, with alternating column support and semi circular apses completing the naves . The building suffered severe fire damage in 1046 . Bishop Azelin planned to erect a new, larger building further to the west and to extend the nave . His successor, Hezilo of Hildesheim, abandoned this plan and instead built on the old foundations, incorporating the surviving walls into the new building . Further important renovations occurred up to the end of the fourteenth century but did not deviate from the ground plan of Bishop Altfrid's basilica . The north and south side chapels date from the gothic period and the tower above the crossing from the baroque period . In the nineteenth century, the original westwork was replaced by a Neo-Romanesque two - tower facade, which stood until 1945 . </P> <P> The Hildesheim Cathedral School (Hildesheimer Domschule, de), which had rooms in the cloisters, was one of the most significant educational institutions of the Ottonian and Salian periods . Its library has served as the Hildesheim Cathedral library (German: Dombibliothek Hildesheim) since 815; it is the oldest library in Northern Germany . </P>

An important ottonian church was built at hildesheim under