Board Thread:Character Discussion/@comment-1170080-20150510131139/@comment-1170080-20150510153250

This could also be true, but it would be more interesting if thats the case. I think that the Real Door leads to an World, that we haven't seen.

And Isaac was trapped in some sort of Pocket Dimension, like in Glindas Case or the Sorcerers Hat, inside the book. The Door was just an Visual Entrance/Exit, to release/trap him.