Check out each of the niches along the corridors; look down the enclosed stairway at the statuary. The vaults overhead are positively stunning. If you exit the cathedral after dark, be sure to take photos of the green-lighted buildings across the street. I took several on automatic exposure with my camera held very steady and still against the building (to avoid a blurred result from slight movement while the shutter clicked), and they are some of the most interesting photos of the city.
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