Let's start with the Basilica Santi Quattro Coronati, not far at all from Colosseo, it was built between 468 and 483 by Pope Semplicio and it's an important witness of the past. The church takes the name from 4 soldiers killed because they refused to kill 4 sculptors who refused to sculp the statue of a pagan god. Late in the evening, nuns still sing vespers.
After such a mystic visit you can go and visit the Basilica di Santo Stefano Rotondo
which is also a very important witness of the past.
Crossing instead the street, in front of you, the "Fontana della Navicella"
and Villa Celimontana, today public park where, during summer season take place lots of jazz concerts but not only. Beside it there is Basilica di Santa Maria in Domnica which is one of the first christian temple of Rome.
Above few pictures of Villa Celimontana.