Isegran is the first place in Fredrikstad mentioned in history. The Earl of Borgarsyssel, Alv Erlingsson, also called MindreAlv, had a small fortress on Isegran in the late 1200s. In the 1670s, the island was fortified with a large battery platform, Isegran tower, and later a small fort was built to protect the river. Until 1685 Isegran was the royal shipyard for the danish-norwegian fleet and during the Great Northern War, the island was the naval base for Peter Wessel Tordenskiold. The historic island can be reached via the Kråkerøy bridge or with the city ferry.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.