For anyone who has done or seen architectural cakes before, you know it's easy to do them badly, as they require more precision than your typical cake.
Here is my rendition, with my template photo in the background:
The cake was formed from 3 6" round cakes with some sculpting. The topper was gum paste. There were many hand piped and painted details-- I tried to capture the spirit of the Duomo without necessarily replicating every detail, as that would have taken weeks. :)
However, I did make 8 tiny flying buttresses for the side of the topper. I then piped on the added details like the arches on the roof.
Because of the octagonal shape of the Duomo, I made many templates for this cake to get it as close to perfect as possible.