LEGO Architecture: Gateway Arch observation deck
This close-up of the observation deck shows the workmanship involved in a piece even of this relatively smaller size.
Photo of LEGO Architecture exhibit by Wally Bahny for TechRepublic
I believe the open areas are actually parking. Odd, but there's parking spaces with cars in them all around the outside of the building. Not sure how they get up there, I always assumed some sort of elevator.