I know this question might seem a bit weird, but this is really difficult to find an answer to me.
Rome is well known to be one of the city in the world which has the biggest quantity of ancient monuments/ruins. It is NOT of course the city where there is the OLDEST monument/ruin in the world. Probably there is some small city in the world where there are pre-hystorical ruins or wall-paintings.
But, within the city of Rome (inside the big round highway called "Grande Raccordo Anulare") which is the oldest monument or ruin which is possible to watch for a tourist?