The main level of the structures wind up under the structures of the Bailey Fort leading eventually to a trap door opening into the fort proper. These structures are used as storage, guard rooms, and an escape route in case of emergency.
There is also a passage that leads up under the Spire, however it lacks an accessway into that structure (at one point there was such an access point, but a team of mechanical assassins used it to gain access to the spire and it was blocked off afterwards). This section contains a secret chamber that in turn has a trap door down to the lower chambers which are used as a secret dungeon for prisoners as well as an underwater escape route for those with the access and the means to travel underwater.
The tunnel leading underwater from these lowest passages proceeds 130 feet further from Merman’s Bluff and into a small cave 20 feet under water.
You can download the map of the substructures of White Crag Fortress from the blog post at http://wp.me/psUVp-2gL