What can you say about the Duomo that hasn't already been said? The exterior is unreal, I felt as if I was looking at something in a video game. It soars above everything else in Florence, wherever you go, it is visible. On the inside, it is just as huge - although bear in mind that most of what adorned it before is in the nearby museum.
Now, the dome. First of all: to get to the top of Brunelleschi's Dome, you have to climb 463 steps - no lifts here. These steps are not all one flight (they are broken up into a few stages) but in places they are narrow, steep, winding and a tad closed in; there is no room for people to turn around and go back down. If you don't feel able to climb it, whether due to fitness or claustrophobia or whatever, then for god's sake don't go up!
Now that that is out of the way, if you do feel able to go up it, then do. It is the most incredible feeling to climb ever further up, first coming out at a balcony inside the dome, up some more stairs, and finally coming out at the top to the most fantastic vista. We were there in low season and it was the second busiest place after the Uffizi; I can imagine that in summer it gets a bit hectic. It took longer to get down than to go up, as people simply must have their blurry photos of the inside of the dome from the balcony above.
One thing to note is that entry to the dome/cathedral/museum is NOT included in the Firenze card, it is an extra sum but completely worth the money.
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