Well you'd have to become an apprentice mason to find out.

That was for that other cult, the stonemasons.

Afaik, they led an initiate down and left to find an exit at night without light or something like that. Maybe they went down the stairs on their own, too

It's where Masons were initiated. I couldn't find out much more. I have a friend here who's a Mason and asked him about it. He said he didn't know anything about it. (or wouldn't tell me)

The whole property is very unique.

a deadly education wasn't that enjoyable to me. i don't know why, the end of the book definitely sets up new mysteries and unknowns, but all in all i found the book fairly ehhh. i don't think I'll be continuing the series.

Date of construction: between 1904 to 1911

And then the story about Templar Knights using it?

I heard it refered to as an inverted tower.

Did you see the news about Levithian waking off the coast of Virginia? Let's go climb him bro.