Hovedøya Abbey was a Cistercian founded on 18 May 1147 by monks from Kirkstead Abbey in England on Hovedøya island, and dedicated to the Blessed Virgin Mary and Saint Edmund. A church dedicated to Edmund already stood on the island, and the monks took this over as the abbey church, modifying it to meet Cistercian requirements. The rest of the monastery follow a modified Cistercian building plan, to take into account a small local hill. The church itself is built in Romanesque style; the rest of the monastery was presumably Gothic. During the medieval period the abbey was one of the richest institutions in Norway, holding over 400 properties, including a fishery and timber yards.
Political turmoil during the succession to the throne of Denmark-Norway led to the end of the monastery. The abbot, having supported the Protestant King Christian II, possibly in a bid to gain support in the face of the coming Reformation, came into conflict with the commandant of Akershus Fortress, Mogens Gyllenstierne, who ironically had supported the Roman Catholic Prince Frederick I. In 1532 the abbot was thrown into prison for his political involvements, and the abbey was looted and then set ablaze, thus ending 400 years of monastic activity at Hovedøya. Any hope the order might have had in restoring the rich abbey was dashed 4 years later, when the Reformation swept over Denmark-Norway.
The site was later used as a quarry for stone for Akershus Castle. The remaining ruins are nevertheless among the most complete of a medieval Norwegian monastery.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.