Library.
The Library has for centuries served as the largest stronghold of information among the Third Division, and perhaps even in all of the Seireitei (no one can really say, for sure). Reading and literary materials are housed from the ground floor up to the topmost level, while the archives are hidden deep beneath the ground.
Kidō spells kept the underground archives intact for hundreds of years, and can be accessed only by the Captains and the Adviser of the Gotei 13.
