This place is gargantuan, colossal, amazing, and enormous.
It's one of those places where the tour guides tend to focus on statistics--how many feet, how much marble, many pipes in the organ, etc.
The mosaics are simply astonishing.
According to the tour guide, the baldacchino was an afterthought. Sounds highly unlikely to me. Anyway, the architect of the building wanted the commission but the Archbishop wanted to give the commission to someone else. So the architect submitted three designs under three different names. His designs came in first, second, and third place. So he got to design the baldacchino after all.