Haven’t been to Rome in about 12 years, it was a wonderful place to just wander on foot, history unfolds with every step, the churches are museums, street artists of all types were everywhere, performing and selling art, etc. We used to stay in an apartment just off Piazza Navona, a block from the Pantheon, which is about 4 feet below today’s streets. The stone staircase up to the apt was worn down in the middle of the steps about 4 inches. Don’t know what it’s like there these days.
I suspect that many more old Roman buildings (or parts thereof) are still in use than is thought. :^)