Sorry to try and bring logic into this but
how the *beep* are the demons able to effect a room that is supposed to be protected from them? One of them moves a desk from out of in front of the door and then they start making it drip blood down the walls to wash away the symbols. If the room is demon proof then their powers should not be able to effect the inside of the room.
The worst explanation, in my opinion, was the whole rusted iron. In many folklore and myths iron is used to ward off demons, fae, and other spirits you want to keep away. Really though the fact that it had to be *beep* rusted iron just annoys me.
Also this isn't a criticism against the movie itself, and i know it's needed for the movie to even exist, but who creates these loop-holes for these demons to be able to take over the world. I know it's the same in every movie that there is some reason why they just can't rule now, but seriously demons that not only pissed off god, but also lucifer and yet they found this one loop-hole.