- Joins
- The corridors to the upper floor
- Built
- Out from the wall, unsupported below
- Keeps
- The space beneath it usable
- And
- Can be cut at a single point
The long corridors that join the wings climb to the upper level by what the dossier calls a hanging stair — a flight carried on the wall itself rather than on a solid mass beneath it.
Built that way, a stair takes up no floor below, and the space under it stays usable. It is also, quietly, a defensive choice: a stair that stands on nothing is a stair that can be broken at one point and then leads nowhere.
It is the last piece of the logic that runs through the whole castle. One gate, bent. One passage, covered. One stair, cut-off-able. Every route through the building can be closed somewhere along it.