The l'île de Ré, opposite La Rochelle, was subjected on several occasions to attack from British soldiers. Conscious of the need to protect access to La Rochelle and Rochefort, in 1681 Vauban started strengthening the island's defences by building a citadel and fortified castle at Saint-Martin-de-Ré, on the North coast.
Built on the site of a fortress where construction work had started in 1627, the square-shaped citadel occupies the eastern part of the town. Its defensive system comprises four bastions, three demi-lunes and a counterguard, surrounded by a moat and a covered walkway. It contained an arsenal, food and powder stores, barracks and officers' accommodation. The citadel opens on to the sea via a small fortified port. From 1873 onwards it became a stop-off point for penal colony prisoners on the way to New Caledonia until 1897 and later to Guyana until 1938. Today it remains a prison for more than 400 detainees and is not open to the public.
An example of Vauban's first system adapted to suit a flat site, the construction was accompanied by an enormous fortified enclosure capable of accommodating the island's population of some 16,000 inhabitants, as well as their livestock, and of storing food supplies and forage in the event of enemy attack. In an arc on the land side, there are bastions, orilloned half- bastions and a counterguard. Two monumental gates, the Porte Toiras and the Porte des Campani form the access points. Also surrounded by a moat and a covered walkway, it is in addition encircled by an open-plan glacis, sloping outwards from the ramparts within canon-firing range.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.