I’m assuming you found that out from doing an actual base defence correct? It never came up during my BB4 campaigns so I never got to experience it unfortunately. What about unused tiles? Are they just corridors or is there no door to go through? Does every room have a connecting door to adjacent rooms? Is there a corridor between connecting doors or are they just doors?
Maybe I’m just overthinking it but I don’t wanna be surprised by own base layout on the first defence mission I have to do.
Yes.
There is rock. No door to that space. Overall base is 2 level map. Lower level are corridors wide enough to allow vehicle movement. Upper level are facilities. Vehicle Bay is exception where there is no upper level - only ladders with small balconies to adjacent facilities. There can be corridors drilled in rock, without actual facility above them but they look different than unused tiles.
Yes, and only doors.
Maybe there will be tutorial about base defence.
Thanks for the info Yokes, appreciated! 
That would actually be quite helpful for new players. I guess we’ll see if they decide to add that in as part of the tutorial or not.