Island in the Baltic Sea to the north of Bornholm; 0.25 sq. km, 102 inhabitants (2001). This rocky island was used as a fortress in the 17th century, and is still administered by the Ministry of Defence, but does not have any military tasks. In summer, many one-day tourists visit the island. In the 19th century, Christiansø was used as state prison. Together with the neighbouring islands of Frederiksø, Græsholmen and some rocks, Christiansø forms part of the archipelago called Ertholmene (literally meaning 'pea islands').