The cathedral is beautiful both inside and out, but make certain you go to the top on a sunny (or sunny-ish) day since the views of Florence and the Tuscan countryside are phenomenal. If you're claustrophobic or have a fear of heights, you might want to avoid the climb to the top, however. There is literally just one way in and one way out, and the staff contribute nothing to society so don't expect any semblance of guidance, so you might end up stuck on the tiny staircase leading up to the top for a good 10-20 minutes or so. And while I waited, I couldn't help but think that, since the staircase scales the wall of the dome, there was literally nothing but air underneath my feet. :) Also, needless to say you need to be relatively healthy to make the climb since there are hundreds of steps.
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