I’ve only heard it mentioned a few times. The only photo that I ever remember seeing is of a narrow wooden raceway used to direct animals. We likely aren’t talking about an entire level, but just a few small, rather narrow low ceiling corridors. Its likely that only one or two platforms were ever used for this purpose. I also remember that it was mentioned that theses remains had historical protection. There doesn’t seem to be much of any mention anywhere on the web, so I’m also at a bit of a loss for details here.
Edit: Here it is. I think that this is the photo that I saw.