Belmont Castle was built in the 10th or 11th century for the Freiherr von Belmont. The first recorded member of the family was Lutefridus de Belmonte in 1137/39. The Belmont family was related to a number of other Raetian noble families including the powerful von Vaz. By the 13th century they were one of the most powerful families in Graubünden. They owned castles and villages throughout the region. Konrad von Belmont was the Bishop of Chur from 1273 until 1282. In 1267 Heinrich von Belmont was the first student from Raetia to study at University of Bologna. By 1200 the Belmonts appear to have moved to another castle near Castrischand then to Tuma Casti by Domat/Ems, probably leaving a bailiff or vogt at Belmont.
In 1352 Ulrich Walter von Belmont led a successful uprising against the Counts of Werdenberg-Heiligenberg. Together with the armies of the Montalt and the Rhäzüns families they defeated a Werdenberg-Heiligenberg army at Mundaun near Ilanz. In retaliation, soon thereafter, a Werdenberg army burned Ilanz to the ground. It is unknown whether Belmont Castle was also attacked and destroyed or was bypassed, but by the late 14th century it had fallen into ruins. On 11 July 1371 Ulrich Walter von Belmont died childless and the Belmont lands were inherited by the Montalt and Sax-Misox families. In 1380 the Sax-Misox family traded the ruins of Belmont Castle as well as other lands to Brun von Rhäzüns. After the trade the castle remained abandoned and continued to disintegrate. From 1932 through 1936 the ruins were excavated and repaired.
The castle was built near the, now abandoned and vanished, Walser village of Fidaz near Flims. It was built on a rocky spire with sheer cliffs on the north and east sides. Very little is known about the construction history and layout of the castle. However, there were two semi-circular ring wall that wrapped around the east, north and north-west sides of the peak with the inner one built first followed by the outer later. The walls connected at least three buildings. The central and eastern building were probably residences. In the center of the half-circle is a 5.7 meters deep cistern. On the southern side of the castle is a smaller rocky peak that stands about 8 m above the castle and has a flat top that is about 10 m × 30 m. No stone ruins have been found on the small peak, so it was probably topped with one or more wooden buildings.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.