If you are playing with buildings that have a footprint of 6" or smaller (advised in the rules" then having no entry points seems the best option, that is how we play. Otherwise if you are going for a theme around a building such as a big factory, I do agree that a minimum of three is the best. Possibly allowing windows as an option and adding the rule that 1 action to break a window or open, close a door could add to some interesting moments.