In 748 Mondsee Abbey was founded by Odilo, Duke of Bavaria. The abbey tradition was that the first monks came from Monte Cassino in Italy. In 788, after the fall of Duke Tassilo III, Mondsee became an Imperial abbey and over the centuries acquired extensive property. Around 800 the Codex Millenarius, an illustrated Latin book of the Gospels was written at the abbey. In 831 King Louis the Pious gave the monastery to Regensburg Cathedral.
It was not until 1142 that it regained its independence, under Abbot Conrad II, otherwise Blessed Conrad of Mondsee. Conrad, formerly a monk of Siegburg Abbey, had been abbot of Mondsee since 1127, and was extremely successful in defending and regaining the rights and possessions of the monastery, to the extent that in 1145 he was murdered by a group of nobles at Oberwang nearby. He was venerated as a martyr and declared Blessed.
Conrad was succeeded as abbot by Blessed Walter of Mondsee (died 17 May 1158), long remembered as a model by the community for his exemplary striving after virtue. He was buried in St. Peter's chapel in the abbey church.
In 1506 possession of the Mondseeland passed from Bavaria to Austria. In 1514 Abbot Wolfgang Haberl established the abbey grammar school. After a period of decline during the Reformation and the consequent disturbances, the abbey entered a new period of prosperity. Under Abbot Bernhard Lidl (1727-73) and especially in connection with the celebration of the thousandth anniversary of the foundation, there was extensive re-building of the church and the monastic premises. From 1773 the abbot was Opportunus II Dunkl, who was the last abbot of Mondsee: in 1791 the abbey was dissolved by Emperor Leopold II.
During the Napoleonic period the Mondseeland reverted to Bavaria for a few years. During that time, in 1810, the Bavarian Field Marshal Prince Karl Philipp von Wrede acquired the abandoned monastery (along with the nearby abbeys of Suben and Gleink), and used it as a castle. Wrede remained the owner even after the return of the territory to Austria and significantly developed the locality, for example by the construction of roads and the establishment of local cheese production. In 1905, on the death of Princess Ignazia von Wrede, Mondsee passed to the Counts Almeida, whose descendants sold it in 1985.References:
First record of Kastelholma (or Kastelholm) castle is from the year 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to the queen. The heyday of the castle was in the 15th and 16th centuries when it was administrated by Danish and Swedish kings and stewards of the realms. Kastelhoma was expanded and enhanced several times.
In the end of 16th century castle was owned by the previous queen Catherine Jagellon (Stenbock), an enemy of the King of Sweden Eric XIV. King Eric conquered Kastelholma in 1599 and all defending officers were taken to Turku and executed. The castle was damaged under the siege and it took 30 years to renovate it.
In 1634 Åland was joined with the County of Åbo and Björneborg and Kastelholma lost its administrative status.