Vestibul

The round chamber was the formal approach to the emperor's private rooms, and the circular stone together with the opening above gives it acoustics that carry…The round chamber was the formal approach to the emperor's private rooms, and the circular stone together with the opening above gives it acoustics that carry. A cappella groups sing in it for exactly that reason: the sound returns cleanly instead of dying in corners, because there are none.

The emperor's bedroom sat directly over it, and the story attached to the space is that those acoustics served him the way they now serve singers, as notice of anyone coming up. The hole overhead was read at the time as a gap between the gods and the man below.