Château de Noirmoutier is very well preserved and a fine example of 12th century medieval architecture. The first traces of the castle appeared in 830 with the construction of a castrum by the abbot Hilbold, from the monastery of Saint-Philbert. It served to defend the monks and the island's population from the Vikings. The castle was rebuilt in stone in the 12th century by the feudal power who was trying to stabilise the region, notably by preventing Norman pillaging. The island at that time was under the control of the barons of La Garnache. The keep was built by Pierre IV of La Garnache, then an enclosure equipped with towers was built around the lower courtyard. In the 16th century, the castle was held by the La Trémoille family, then viscounts of Thouars.
The castle has resisted numerous attacks, like the English in 1342 and 1360, and again in 1386 under the command of the Earl of Arundelthe Spanish in 1524 and 1588. In 1674 it was taken by the Dutch troops of Admiral Tromp.
Château de Noirmoutier was sold in 1720 to Louis IV Henri de Bourbon-Condé who resold it in 1767 to Louis XV. During the French Revolution, the castle served as a military prison. During the 19th century, the castle was used as a barracks. In 1871, during the Paris Commune, insurgents were imprisoned there. In 1960, a house was built within the castle grounds by the governor of the island and the castle. Today, the keep houses the Noirmoutier Museum.
The keep at the centre of the castle is solid and rectangular. Built of rubble, it has three floors with the lords' residence at the top. The keep has numerous murder holes and defensive turrets at the corners. The rectangular fortification consists of two towers, a single gate and two watch turrets in the four corners. At the beginning of the 18th century, the towers were reconstructed and the keep adapted for artillery.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.