Kastellholmen is an islet in the centre of Stockholm, Sweden. It belongs to the district of Skeppsholmen. It is connected to adjacent Skeppsholmen through the Kastellholmsbron bridge. Kastellholmen has an area of 31,000 m2. Kastellholmen has previously been known as Notholmen, Lilla Beckholmen and Skansholmen. On the island there is a small castle, Kastellet, which was built between 1846 and 1848 under design by Swedish officer and architect Fredrik Blom.