OK the passages lead to the Dungeon, so cold and damp. Not a nice place to visit let alone live there, and it has none of the comforts of home.
The passages access almost all of the rooms in the castle and one can only ponder over all the possible uses. The Map Room, Commodore Bourne's personal study, was originally only accessible through the passages.
Looking closely one may notice this painting but...
... what the heck ... who are they and what are they looking at?
Why they have a clear view of the Drawing Room!!!