From the late 16th century onward, the Abbey of St. Ulrich and St Afra was one of the 40-odd self-ruling imperial abbeys of the Holy Roman Empire and, as such, was a virtually independent state. The territory of that state was very fragmented: the abbey of St. Ulrich and St Afra proper enclaved within the Free Imperial City of Augsburg, and several small territories disseminated throughout the region. At the time of its dissolution in 1802, the Imperial Abbey covered 112 square kilometers and had about 5,000 subjects.
The Benedictine monastery was preceded by an original foundation established at an uncertain date, but at least as early as the 10th century (and in its turn quite possibly a refoundation of a still earlier one from the 5th or 6th centuries), by the Kollegiatstift St. Afra, a community of the priests charged with the care of St Afra's Church (now the Basilica of Saints Ulrich and Afra), where the relics of Saint Afra were venerated, and next door to which the community premises were built.
Between 1006 and 1012, Bruno, Bishop of Augsburg, removed the canons to the cathedral chapter and gave the premises to Benedictine monks whom he brought from Tegernsee Abbey, thus turning it into a Benedictine monastery. It was granted Imperial immediacy as an Imperial abbey in 1577, but this status was bitterly contested by the bishops of Augsburg, and the legal conflict was resolved in favour of the abbey only in 1643/44.
The abbey was dissolved in 1802 during the secularisation of Bavaria. The city of Augsburg and the state of Bavaria divided its territory between them. The monks however were permitted to remain in the premises of the dissolved monstery. In 1805 a French military hospital was installed here; after six monks, including the abbot, had died of infectious diseases, the remainder moved into a private house. The hospital was replaced in 1807 by a Bavarian cavalry barracks, known as the Ulrichskaserne.
The barracks remained here until World War II, when in 1944 the buildings were largely destroyed. The remains were not cleared until 1968–71. On the site the 'Haus St. Ulrich' has stood since 1975, an academy and pastoral centre of the Diocese of Augsburg. The sarcophaguses of Saint Afra and Saint Ulrich are preserved in the crypt.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.