The Citadel isn't just a backdrop. There's a whole history behind it: the Framework Assemblers who built it, the Derailment that broke the machine hierarchy, Mainframe and its strata, and the powers fighting over it, Ganden and Tzhal included.
What have you pieced together from the official world archive, item descriptions and the game itself? Theories welcome, just say what's confirmed and what's speculation.
Read more: the powers in the codex (Ganden, TZHAL, Clementines, Teks), the glossary for mathrons, daemons and the rest, and the official world archive at thecitadel.com/world.