This is the Research and Development Department. It is no longer part of the Gotei, but Mayuri is still captain. This is where you go if you wanna be disected-erm find something you need to be resarched.
This is where all the Shinigami reside. There is a vast amount of space in it, so much so that it could take a day for the old 13th squad to get to the first division. Around the whole Sereitei is Seki Seki which keeps people out effectively except for the four gates leading into, that are guarded.