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 Akershus Fortress in the 17th century.