Vince was right.
(Daniel's legs in the upper part of the photo
indicate the photographer's position near Vince.)

The opening just below Daniel's feet
is 9 feet tall and will soon be fitted with a 150 year-old door
that looks like it came straight out of Middle Earth in The Lord of the Rings.
This door will be surrounded by bookshelves and will lead into a private study.
The semi-circle you saw in the first photo is the floor of this hidden room.