Below the floor and pews of the Grand Cathedral, there is a network of tunnels (A) used to quietly and discretely work special effects and illusions within the cathedral itself. These tunnels wind and twist to keep lights within from exposing them to view from the pews above, as well as to support the floor without betraying that there are tunnels below. At the center of the network (B) is a ladder up into the central statue from which the actor portraying the idol could speak or dispense smoke and light as needed. An area at the back (C) was a workshop for preparing some of the special tricks of the cathedral.
Closets full of materials for the show in the cathedral (D) are still somewhat stocked with flash powder, incense, and the like. One of these closets was bricked up long ago. The reason was lost long ago, what did they trap inside? Another of these closets has a secret door at the back leading into a secret laboratory (E).
In addition to summoning and sacrifice, this laboratory area has a strongly isolated lab (F) for experiments of magic that required isolation for the experimenter to feel safe. The laboratory office (G) is connected to lower levels by a secret stair.
I went ahead and added the cellar (H) from the Grand Cathedral map where it belongs on this level.