Warsaw Citadel is a 19th-century fortress in Warsaw, Poland. It was built by order of Tsar Nicholas I after the suppression of the 1830 November Uprising in order to bolster imperial Russian control of the city. It served as a prison into the late 1930s, especially the dreaded Tenth Pavilion of the Warsaw Citadel (X Pawilon Cytadeli Warszawskiej); the latter has been a museum since 1963.