What is a secured facility? Here is an example from the 21st century.
The President Emergency Operations Center (PEOC)
The PEOC was built six stories below ground under the East Wing [ed. of the White House] to provide the President with a secure meeting place in the event of an emergency. Access to the PEOC is by an elevator located behind multiple vault-type doors with biometric access control systems.
As a writer, I would start with this as a basis for the room where the meeting would be held. I would do other research and consult experts, and, then, I would envision how an attack might proceed.