Ertholmene, sometimes termed Christiansų, is a smallish archipelago, consisting of the three islands Christiansų, Frederiksų and Graesholm as well as some rocks and skerries in the
Baltic Sea some 18 km northeast of
Bornholm. Ertholmene (literally meaning
pea islands) belong to
Denmark, forming the easternmost part of that country.
In 1684 a fort was built on Ertholmene, which served as an outpost for the Danish navy until 1855. The fortifications and barracks have been changed very little to this day.