When preparing the 3D-models for this set, I was leafing through a couple of books on magic, alchemy, astrology, natural philosophy &c. to get some inspiration and, even more important, to find suitable textures and symbols.
That's where I found the inscription on the wall. It reads:
Byss and Abyss : Nothing and All : Time and Eternity
I couldn't find the word "byss" in the dictionary. I suppose the expression means something like "heaven and hell" or "above and below". Maybe one of you can shed some light on the subject.