https://en.wikipedia.org/wiki/Isosaari

Isosaari (Swedish: Mjölö) is an island in Helsinki, part of the Ulkosaaret district of the city, more specifically of Itäsaaret, i.e. ‘eastern islands’. It was one of the fortified islands of the Gulf of Finland. Isosaari is one of the outermost islands in the Helsinki region, and it is a site that is important from the perspective of cultural history. Its sights and its nature are unique.