Answer:
Vladivostok was once closed because of its military importance, for it was the location of the Russian navy base.
Step-by-step explanation:
In the given passage about Vladivostok, the speaker explicitly mentions why the place had been previously closed to outsiders and even Russians. Even though it had now been opened for the public, the same has not been the case during the period of the Cold War.
Opened by Mikhail Gorbachev, Vladivostok was a "strategic military" location for the Russian Navy and thus, closed for easy access, especially during the Cold War. But more lately, though still the "headquarters of the Russian Pacific naval operations", the base is now open for everyone to visit.