The Citadel is a malicious company founded by Kal. At the time of writing, they are believed to have gone out of business, as nobody has heard of them in a long time.
Their headquarters were located in the Old-Old World, which is now gone because they destroyed the entire world along with their own headquarters, with the help of the substance Red Cataclysm III.
They are responsible for the creation of Gohma, the giant mechanical spider that wrecked havoc in the Old World, but that was defeated by Grape, Hellco. and S.O.R.A. combined. The mechanical spider's remains were kept by S.O.R.A. for research, which sparked controversy.