Hovedøya

Hovedøya is one of several small islands off the coast of Oslo, Norway in the Oslofjord. The island is quite small, no more than 800 metres across in any direction, the total area is 0,4 square kilometre. It is well known for its lush and green nature, with a wide variety of trees, bushes and flowers

The name is from Norse times (Hǫfudøy). The first element is hǫfud 'head' (here in the sense 'hill' or 'height'), the last element is the finite form of øy 'island'. The name is a reference to the top of the hill on the island - at 47 metres it is the tallest point of the inner Oslofjord islands by a good margin. (See also Nakholmen.)

The Cistercian monastery, Hovedøya Abbey, was built on the island, and opened on 18 May 1147. During the Medieval period, the monastery was a leading economical force in the Oslo region. It was, however, closed down before the Reformation after the abbot came into conflict with King Christian II. The military official at Akershus Fortress had the abbot imprisoned and the monastery looted and burned down in 1532. Parts of the ruins of the monastery remain on the island, but much of the stonework was used in the expansion of Akerhus Fortress in the 17th century.

The island has also been used for military installations, and former military buildings and artillery remain on the island. When Denmark-Norway became involved in the Napoleonic war, two cannon batteries were built to defend Oslo and Akershus Fortress in 1808 (again using the monastery ruins as a quarry). Four depots for gunpowder were built in 1826 on the high points of the island to ward off intruders, and a fifth was built in 1867. On the eastern half of the island is a Victorian house, popularly known as the "Laboratory". It was used by Director of Armory Ole Herman Johannes Krag, co-inventor of the Krag–Jørgensen rifle in the late 19th century.

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