Rome’s most singular trait is that it never starts over — it turns the last age’s stones to the next age’s use: temple into church, tomb into prison, arena into quarry. Follow this thread through three POIs and you will see that Rome is really "the same ground layered for 2,800 years."
Temple → church: because it became a church of the Virgin, it is the one ancient Roman temple to survive whole.
Tomb → fortress → prison → museum: four lives of one building, all written into this single structure.
Arena → quarry → shrine: its missing half was hauled off to build St Peter’s.