Alright, I'm sure everyone who's been to the ruins has wtf'd at the codes written on the mysterious pillars. ITT, I want to break this bloody code.
I'm pretty sure that this is a formal single-alphabet substitution. However, based on the frequencies of each character from one floor to the next, I'm certain that all 4 floors use different ciphers. Also of note is that the first pillar you see at all 4 entrances of 1F all say the same thing.
Each letter indicates a different pillar, and each line corresponds to each particular line on those pillars.
A web site I found fairly useful: