I visited the dome of St Peter's one afternoon with my mother - we had to queue for about half an hour to pay €7 to take the lift to the bottom of the dome. From there you go through a door and up a ramp into the base of the dome where you are close up with the wonderful mosaics and can look straight down into the basilica below or up at the rich decoration on the inside.
It was great to be so close to the mosaics - they looked so fresh that they could have been created yesterday. There were these coats of arms above the doorways;
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