In the very heart of the Baltic Sea, in the Ertholmene archipelago, the small island of Christiansoe – less than 0,5 km2 (barely 0,3 square miles) – awaits you. Thanks to its geographic form, which constitutes a natural harbour with the island of Frederiksø, it has been used as such for more than a thousand years. Pirates and fishermen succeeded one another here before the construction of its fortress, built between the 17th and 18th centuries, in the time of the Danish-Swedish wars. Visiting Christiansoe, you see the shaping of great history, the re-emergence of the rivalries and alliances that took place in the Baltic Sea. Today, this peaceful island offers you wild and preserved nature. A natural reserve since 1926, it is home in particular to a very large population of nesting and migrating birds that find food and tranquillity here.