Sankt Pölten Cathedral has been the episcopal seat of the Diocese of Sankt Pölten since 1785, having previously been the church of the Augustinian Abbey of St. Pölten, dissolved in 1784. The building, despite being a well-preserved late Romanesque structure, is Baroque in appearance.
The use of the site for religious buildings is believed to date from around 790, when a Benedictine monastery was established here, to which were brought the relics of Saint Hippolytus, after whom the present city is named. In 828, the monastery became a possession of the Diocese of Passau, and a centre of missionary activity, predominantly in Great Moravia. After the invasion of the Magyars in around 907, the monastery was almost entirely destroyed, and was not rebuilt until after the Battle of Lechfeld in 955. The first documentary reference is in a charter of 976 from Emperor Otto II to Bishop Pilgrim of Passau.
Under Bishop Altmann of Passau the abbey became an Augustinian canonry, which was dissolved in 1784 as part of the Josephine Reforms.
In around 1150, the abbey church was rebuilt with three naves but no transept, with a westwork including two towers. In 1228 Bishop Gebhard changed the dedication, formerly to Saints Peter, Stephen and Hippolytus, to the Assumption of Mary. After a fire it was rebuilt again between 1267 and 1280. After another fire in 1621 the entire building was remodelled in the present Baroque style.References:
Ängsö Castle was first named as "Engsev" in a royal charter by king Canute I of Sweden (r. 1167-1196), in which he stated that he had inherited the property after his father Eric IX of Sweden. Until 1272, it was owned by the Riseberga Abbey, and then taken over by Gregers Birgersson.
From 1475 until 1710, it was owned by the Sparre family. The current castle was built as a fortress by riksråd Bengt Fadersson Sparre in the 1480s. In 1522, Ängsö Castle was taken after a siege by king Gustav Vasa, since its owner, Fadersson's son Knut Bengtsson, sided with Christian II of Denmark. However, in 1538 it was given by the king to Bengtsson's daughter Hillevi Knutsdotter, who was married to Arvid Trolle.
In 1710, the castle was taken over by Carl Piper and Christina Piper. Ängsö Castle was owned by the Piper family from 1710 until 1971, and is now owned by the Westmanna foundation. The castle building itself was made into a museum in 1959 and was made a listed building in 1965. It is currently opened to visitors during the summers.
The castle is a cubical building in four stores made by stone and bricks. The lower parts is preserved from the middle ages. It was redecorated and expanded in the 1630s. The 4th storey as well as the roof is from the expansion of Carl Hårleman from 1740-41. It gained its current appearance in the 1740s.