closed city

closed city (plural closed cities)

1. Any of several settlements, especially in Russia, that have travel and residency restrictions, typically because they house sensitive nuclear or defense facilities.
   • 2017 January 3, Douglas Preston, The Lost City of the Monkey God: A True Story, Grand Central Publishing, →ISBN:

     [A] man with a gravelly, slow-talking, New Mexico drawl, Glassmire had built a career as a respected scientist who had worked as an engineer for Los Alamos National Laboratory in the mid-fifties, when Los Alamos was still a closed city.

• cyclotides