Some times when I work on something, I like to give it a story to add character. In this case, the above ground entrance to a underground shelter of some sort is sealed due to a zombie infection outbreak that happened deep inside. I can see this being in the middle of a heavily inhabited city on an imperial planet where it has been pretty much overlooked since it was sealed up with all of those poor people still inside. People soon forget why it was sealed up after a while, despite what sounds might still be hear coming from behind the door or through the air vents....