Closed Cities

A very Soviet phenomenon, but one still existing in modern Russia, although many cities are now "open". Known as ZATO (zakrytye administrativno-territorial'nye obrazovaniia) in Russian.

Appeared from the late 1940s onwards.

These are entire cities that foreigners cannot enter and Russians need a permit to live there, being subject to movement restrictions. Some were physically surrounded by barbed wire, with armed guards. They were referred to only by a postal-code and did not appear on Soviet maps. They were located in remote areas. These cities were usually there to serve the needs of defense industrial complexes or classified research institutes.

Note that living there was not necessarily bad - it involved (and to a lesser extent, still involves) reasonably good and prestigious if secret work, lots of cool if sometimes dangerous stuff happening, and various privileges such as a temporary exemption of local businesses from taxes in the Nineties.

Other closed areas existed elsewhere in the Warsaw Pact, especially on the border between West Germany and East Germany.

Forty-four Closed Cities are acknowledged to exist as of 2021 in Russia, but around fifteen more are also believed to exist.

Examples of Closed Cities include:

Film

In The Sum of All Fears movie, agent Cabbot speaks about the closed city called Arzamas (most probably referring to Arzamas-16, now Sarov).

Literature

In Devil May Care Julius Gorner attempts to destroy Tryokhgorny, referred to by its postal name Zlatoust-36.

Live-Action TV

An episode of Airwolf, "Proof Through The Night", is set in Sverdlovsk.

Video Games

Possibly non-Russian example: City 17. It wasn't closed by them anyway.
The game Gorky 17 (a.k.a Odium) takes place in the eponymous closed city (note the error in transcription, as there should be a dash between the name [Gorky is an actual city] and a number).

Real Life: Known Closed Cities

Severomorsk. Administrative base of the Russian Northern Fleet. Still closed.
Gorky, now open and back as Nizhny Novgorod.
Tryokhgorny. Still closed.
Mirny, Arkhangelsk Oblast. Still closed, it is home to the Plesetsk Cosmodrome.
Verkh-Neyvinsk, aka Sverdlovsk-44. Still closed.
Snezhinsk, Chelyavinsk Oblast. A nuclear research center.
Sverdlovsk, now Ekaterinburg. Not a proper closed city, but it was still practically closed to foreigners because of its status as an industrial center. It was not the only such borderline case.
Mezhgorye, a closed city near Mount Yamantau that hosts some 17,000 residents that work on defence facilities around the mountain.