Grynau castle tower is situated at the then only bridge over the Linth river. Built probably in the early 13th century by the House of Rapperswil, the castle secured the strategically important river crossing in the area between the Grafschaft Rapperswil and the House of Toggenburg. The property was documented in 1311, when the castle was taken by force by Rudolf von Laufenburg-Rapperswil probably from the Toggenburg family.
In the summer of 1799, the French and Austrian troops fought in the Second Coalition War at the strategically important bridge which was destroyed three times, and rebuilt, and occupied by the French troops in the aftermath of the Second Battle of Zurich. Again in 1833, Swiss federal troops were concentrated at the Grynau castle, on occasion of the then planned division of the canton of Schwyz, however, waived without an armed intervention. And again in a Swiss civil war, the so-called Sonderbundskrieg, federal troops crossed the important bridge in March 1847, without a single dead soldier on both sides.
In the 19th century the remaining buildings, the tower, the adjacent barn and the former accommodation building, were bought by Schlossvogt Kälin, who rebuilt the surrounding building into the Landgasthof Schloss Grynau, a country inn, which is still held by the family.
The architecture dates back to the early 13th century. The five-story tower measures 12.5 x 12.5 metres, the foundation walls are 2.2 metres meters thick. The castle was between 1807 and 1816 widely rebuilt on occasion of the construction of the Linth channel; the road and bridge cross as of today in between the preserved tower and the former economic structures that were widely broken. In 1906 a fire broke out in the barn, which was adjacent to the tower and destroyed the roof and the interior of the tower. The castle tower was re-decorated and re-roofed and a new barn built in the following year.
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.