Board Thread:Spoilers!/@comment-26125643-20151012001020/@comment-25926288-20151013135827

Well, Merlin is the most powerful of them all. Maybe he thought once he got out the tree he wanted a nice house. Or maybe that's where his astral projection lives, lol. Could have at least let the Apprentice live there.....

The door was summoned with the Apprentice's magic, but it could still be there though.