how might the whole story be read as an allegory of a jorney to the human mind?

The character of Roderick is pretty unstable. This is a Gothic tale so mental instability works well. Madeline was also unstable, infact the whole Usher family was pretty much crackers. The only sane person to enter the story was the narrator but he seems to succumb to Roderick's bizarre behaviour. So, in the end, this story is a journey into crazyville. If you don't get caught up in Poe's twisted tale it becomes doubtful if Madeline even existed or not. Is she merely a reflection of Roderick's anxiety and neurosis? I'm not sure but if anything, Poe at least gives us a great camp fire story!