A Pushmi-pullyu in Middle Earth!?!

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Much depends on the door mechanism and handles, doesn't it?

The door on the other end of the room, the one that Gandalf put a shutting spell on, had metal rings as door handles, which makes me assume the door in question did too. This should mean that there was no mechanism to keep the stone door closed unless the lock was turned, obviously a bad thing for the defenders. If the door opened outwards however, and they had metal bars, swords, spears or anything long enough, they would probably have been able to block the door by threading it/them through the iron ring and past or preferably across the doorpost. Then the door would have been very hard to open from the outside, as you can't get much force pulling on a handle, even if you are a cave troll, and able get your bulky fingers inside the ring. Who says that cave trolls have fingers anyway? Then don't have toes!

As the door did open inwards the attackers could apply a lot more force on it by ramming it. Unlike pulling on a handle, this can be a collective effort too, although I suppose the cave troll just kicked it in.