1. The Old City of Jerusalem — Jerusalem
Three thousand years of continuous building inside one square kilometre.
Jerusalem's walled Old City holds the Western Wall, the Church of the Holy Sepulchre, the Dome of the Rock and Al-Aqsa within a single square kilometre — the densest concentration of religiously and historically significant architecture anywhere on earth.
The walls themselves are Ottoman, built by Suleiman the Magnificent in the 1530s on much older foundations, and the Cardo running through the Jewish Quarter is the Roman main street, excavated and partly restored.
It works best as several visits rather than one: the Western Wall tunnels, the ramparts walk along the top of the walls, and the four quarters each on their own terms. Modest dress is required at the religious sites.
Good to know: Walk the ramparts on top of the walls — few visitors do it and it reframes the whole city.