Behind the locked door. . .

It's been a while since I've read the book, although I just watched this again last night, but I think it was just another way in to the cellar, which, seeing the state of the lock at the end of the movie explains his appearance in the window. I just assumed that he had cut the lock prior.

That door was just another entrance/exit to the barn cellar. George had padlocked it to keep the boys from playing down there. "Holland" was mad that the door had been locked, because it was supposed to be part of the magic trick that the "twins" were planning to perform.


He who conquers himself is mightier than he who conquers a city.

The padlock was put on the door which was the main entrance to the barn cellar. The only other way into the cellar was the trapdoor in the barn. It is established early in the film that their uncle has padlocked the door because he doesn't want the kids playing in there, seeing it was here that the boys' father perished. However the twins just use the trapdoor. The problem is, that doorway was going to allow Niles to sneak back up into the barn and audience after "disappearing" through the trapdoor during the magic show. WIth the door locked, how was the magic trick going to be performed?
If you listen closely before the storm sequence, when Ada is looking for Niles, you can hear him sawing away at it when she enters the barn. She calls down to him and he comes up through the trapdoor, however he had been sawing SOMETHING (the lock). At the end of the film, when

SPOLIER


Ada throws herself and the lantern into the cellar, she is assuming there is no other way for Niles to get out. Obviously she is aware of the lock. However, unknown to her, Niles has sawed the lock off in preparation for the magic show. Again, listen carefully after Niles yells "NO!!" when he sees Ada about to throw herself and the lantern into the cellar. You can clearly hear the sound of a door opening --that is Niles making his "escape".