All good and safe... except a determined person with a hammer could get through the wall in minutes, or pull up the floorboards/roof and come in that way.
The door is only as strong and the weakest part of the building/bunker. Why spend days breaking through a "vault" door when theres maybe only a brick wall 6 foot away.
The best defence is being invisible, hard to beark in if they don't know you exist in the first place.