A closed city or town is a settlement where travel or residency restrictions are applied so that specific authorization is required to visit or remain overnight. Such places may be sensitive military establishments or secret research installations that require much more space or internal freedom than is available in a conventional military base. There may also be a wider variety of permanent residents, including close family members of workers or trusted traders who are not directly connected with clandestine purposes.

Sarov is a closed town in Nizhny Novgorod Oblast, Russia. It was known as Gorkiy-130 (Горький-130) and Arzamas-16 (Арзама́с-16), after a (somewhat) nearby town of Arzamas, from 1946 to 1991. Until 1995, it was known as Kremlyov/Kremlev/Kremljov (Кремлёв). The town is closed as it is the Russian center for nuclear research. Population: 92,047 ; 87,652