You can't walk around the center of Florence without being enthralled by the enormity of the Cathedral of St. Mary of the Flower (Cattedrale di Santa Maria del Fiore). She is the 4th largest cathedral in the world (we saw another top 4 in Milan last month!).
Begun in 1296 and complete in 1426, it is an architectural marvel. For all the skinny on builders, designers, etc, you should read the wikipedia link above.
We overheard more American accents on the perimeter of the Duomo grounds than anywhere else we've been to in Europe. Must have been a zillion high school and university kids (yes... less than 1/2 my age) buzzing about.
We did not wait in the ginormous line to go to the top of the dome... and its thousands of stairs, but did enjoy the inside of the cathedral. After the ornate style of Malta's beautiful cathedrals, the interior felt stark and plain, the Gothic era did not adorn its cathedrals like those of the Baroque period. But please don't look too far into that... as I about went cross eyed trying to educate myself on the different style of interior cathedrals. So on to photos...