“Shut the door!” he begged them, “I fear that dragon in my marrow. I like this silence far less than the uproar of last night. Shut the door before it is too late!”
Something in his voice gave the dwarves an uncomfortable feeling. Slowly Thorin shook off his dreams and getting up he kicked away the stone that wedged the door. Then they thrust upon it, and it closed with a snap and a clang. No trace of a keyhole was there left on the inside. They were shut in the Mountain!
And not a moment too soon. They had hardly gone any distance down the tunnel when a blow smote the side of the Mountain like the crash of battering- rams made of forest oaks and swung by giants. The rock boomed, the walls cracked and stones fell from the roof on their heads. What would have happened if the door had still been open I don’t like to think.

Chapter 12 Inside Information