The availability of doors and stairs - it was a long time ago, three years ago. Since then, they have changed the mechanics, and one of the corners of the base building became the target of the purge. Doors and stairs are completely unimportant now, open all doors wide open - no one will enter inside if the purpose of purge is the corner of the building next to the door. It is to the corner that NPCs will strive, and having made their way to it, they will destroy it.
When the announcement of the purge appears, see on the map at the maximum scale which corner of the base is marked with a cross, and see if the passage is open to it. The passage through the base will also be considered.