[alatar]

I understand that one's fellows would have to step back as the door is being opened, but from the defenders' side, how would you keep the door closed?

The door opened inwards, and so if you wedge it, the wedges would become tighter the more the opposition pushed. Given the right circumstances, you could put something across the door (assuming you have something to hold the bar) to keep it closed as well.

Think that Boromir does this in Peter Jackson's movie.

Also, the door could be braced from the opposite wall, or using the tomb.

None of these methods work if the door opens outwards.