Down from the secret entrance chambers is one of the early draws to build this facility: a mine underground but still open pit style (B) to obtain the valuable ore found here (e.g. mithril, orichalcum, palladium). Since its construction, a digging creature has opened a way into the pit (E).
While overseers and miners were originally housed off the chamber (C) and in an adjacent hall (A), eventually other passages were added (F, G, H) to auxiliary chambers as the location grew from a mine to much more. As the passages descended, the command center (D) for the facility remained on this level for some time.
Expect a lot more maps in this facility. Not even I yet know where all the stairs and ladders I’ve drawn in lead to :).