The Château de Rosanbo, overlooks the Bô river valley. The origin of its name stems from this fact, as in Breton it means 'rock on the Bô'. The château, which was in the past the stronghold of the Coskaër de Rosanbo family, then later of the Le Peletier de Rosanbo family, is square in shape and has been developed and re-fashioned throughout its history. In the 14th century, a fortified castle was built on a strategic headland, located 4 miles from the bay of Saint-Michel-en-Grève in order to prevent the ascent of the Bô by invaders. In the 16th century, it was extended with a Gothic manor.
The buildings were further extended in the 18th Century by Louis Le Peletier de Keranroux, first president of the Paris parliament and husband of Geneviève de Coskaër (the last inheritor of that name). At the time of this redevelopment, the architect Joubert added a gallery to the château and a corner room overlooking the valley. The old kitchen was transformed into a wood pannelled dining room and, on the floor above, additional new space allowed small rooms such as private bathrooms and boudoirs to be created. At the end of the 19th Century, new suites were designed by the architect Lafargue, who was also responsible for the restoration of the châteaux of Josselin and Chenonceaux.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.