It is interesting that the choice of words used to describe the way to reach Roderick's studio is that of
many dark and intricate passages.
It is interesting, because the imagery that may come to the mind of some readers is that of the human brain itself, which is a...
massive number of network and sub-networks of interconnected cells that serve as pathways of information.
The metaphor here is that, in Roderick's house, these dark and intricate passages lead straight to Roderick. Hence, the idea is that, in order to get to the mindset of such a conflicted and complex individual, one must be willing to enter a dark and complicated mental journey. Such is the journey that the main character endures; one in which the world, as he knows it, becomes convoluted. His beliefs, ideas and concepts about life, death, love and even nostalgia are questioned by the actions of strange Usher, and by the way that things are in the entire household.
Therefore, the dark and intricate passages indicate that such is the state of mind of Roderick Usher; that it is hard to get to the center of who he is, because he is more of an entity that exists within an eternal, deep conflict with himself and his environment. His life is dark. His spirit, state of mind, and his entire sense of being is intricately confusing and enigmatic. All is too murky and dark to really be able to see any light at the end of the tunnel that is Roderick's life.