The Fourfold Keep

From the introduction:

While hints and traces of the legendary Mages Four can be found throughout Dragon’s Delve, they come to their culmination here on Level 15 which, more than 800 years ago, served as the home and base of operations for the group. Like the Gauntlet Arcane on Level 14, nothing is known about the purpose or contents of the Fourfold Keep prior to the mages’ arrival. Indeed, it seems likely that even the physical layout of the level was altered to suit their preferences, so the ancient history of this section of the dungeon will likely never be uncovered.