Created by G.M. Armfelt on a group of six connected islands (Susisaari and Iso Mustassari) and islets, the fortress with its bastion advanced in the roadstead, boasted around 5 000 inhabitants at the end of the 18C, making it the second largest city in Finland, after Turku and ahead of Helsinki. When Russia invaded Sweden, the protection it could offer was minimal. Fifty years later, bombarded by the Franco-Anglo squadron during the Crimean war, the fortress was seriously damaged, demonstrating its limitations in the face of more modern weapons.
Today, the Islands offer a pleasant place to relax for those who wish to stroll through the galleries, to doze on the grass in the shade of the fortifications, or eat in the Walhalla restaurant located in the blockhouses. The fortress is a UNESCO listed world heritage site.