Complex sentence: From 1818 to 1828 and in 1860, the fort was strongly enhanced; but its military significance diminished after the First World War, and in 1934 it was sold to the Copenhagen harbour services.
Split into simpler sentences: From 1818 to 1828 and in 1860, the fort was strongly enhanced, but its military significance diminished after the First World War. In 1934 it was sold to the Copenhagen harbour services.