Region
Main Hall map
Rooms the sources name
Sub-areas attached to the Main Hall are the Entrance Corridor, Boudoirs, Ballrooms, Elevator Tower, Waterworks, Torture Chambers, Servant's Quarters, Storerooms, and the Archives. IGN's mirror list places several Mirrors of Life across those rooms. I am not reprinting a pin for every one, because the list and the room count do not line up cleanly in a single table.
Brauner's Atelier is here. The Artist fight is listed at 4,250 health. A phantom of Isaac stands on the Castle Drawbridge at 1,500 health and does not drop a card. A door by the Archives leads toward Alucard's Tower. One of the four sage sigils is in the Archives, on an upper floor, with a lever puzzle on the left.
Secrets that are actually located
The secrets worth a detour are the sage sigil, the Alucard portraits and the tower beyond the blue flames, Brauner's paintbrush drop, and the Notre-Dame painting in the Gallery of Eternal Night. A generic 'all main hall secrets' list is not in one source, so this page is that short set and nothing padded onto it.
What I ran into
I merged four separate notes — the sigil guide, the Alucard writeup, the Brauner boss page, and the phantom Isaac row — into one room list. If a secret was only a keyword and not a room, it stayed off the page.
Questions
Where is the Main Hall sage seal?
In the Archives, upper side. The Hall of Mirrors page has the lever note and the other three seals.
Where is Brauner?
Brauner's Atelier in the Main Hall. He is The Artist.