Bischofszell castle was built in the 12th/13th century. It was damaged by the city fire in 1419 and repaired soon thereafter. During the 17th and 18th century the castle was expanded and totally renovated. The newly founded Canton of Thurgau took over the castle in 1798, but sold it in 1811 again. About 1838 the west part collapsed, and in 1843 the keep was broken up. Since 1930, the castle has been owned by the municipality.
The old bridge, constructed in late gothic style, was inaugurated in 1487. It is the oldest bridge in the entire region.
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.