Duomo di Milano

Stand in the square a good while before you go anywhere near the door. Pale marble and a whole forest of spires - there are around 135 of them - against a hard blue sky, and it's far bigger than any photograph makes it look. They started it in 1386 and were still finishing details into the twentieth century, which you can feel. Go up onto the roof if you do one thing: you walk out among the spires themselves, which is completely unlike looking at it from below.