Obviously, some of the details are a secret. However, as I understand it, the mountain complex is a buried bunker protected by a great many tunnels and large blast doors. Perhaps it is effectively impenetrable.
For the bunker to be useful as a command post, however, there have to be working radio antennas on the surface to communicate with the rest of the U.S. military. There’s probably buried communication cables as well, but how deep could they practically be buried? You cannot dig a tunnel that is miles deep all the way from the bunker to the missile fields in Nevada, it has to surface somewhere.
Anyways, while we the public don’t know the details, it seems highly unlikely that the KGB did not obtain most of the schematics at one point or another. They must know where all the surface antennas and buried communication cables are.
Thus, I wonder : what would have stopped them from just nuking the antenna clusters and cratering the communication cables? I’ve heard rumors of a buried communications missile they could launch, but even that thing must be in a silo with an armored door that could also be nuked.
Also, even if the soviets couldn’t destroy the bunker directly, what would have stopped them from simply collapsing the entrance tunnel? How would the bunker personnel have ever gotten out if that happened?