Citadellet (The Citadel) is the name of a 19th-century fortress built between 1848 to 1851. The fortress was intended to protect the Royal Norwegian Navy's Karljohansvern naval station in Horten. Its architect was Christian Heinrich Grosch (1801-1865). The fort was demolished in 1971 to make room for a new large welding hall to be added to the naval ship yard.