On the far side of the Forum, heading south , are some of Rome’s most ancient sights, remnants of the time when this area was mostly green, bucolic countryside. A lake lay where the Colosseum stands now, drained by a small stream that wove between the Palatine and Celian hills, curving to empty into the Tiber close by the Circo Massimo. The slopes of the Palatine and Celian hills were inhabited by people living in shanties and huts, until the great fire of 64 AD, when Nero incorporated the area into his grand design for the city, building a gigantic Nymphaeum to support his planned gardens on the Celian Hill, part of his Domus Aurea, and cleaning up the slopes of the two hills. Eventually a temple to the deified Claudius was built on the Celian Hill, and the Palatine became the residence of the emperors, fed by water brought by the arched span of the Aqua Claudia. 

Nowadays, this part of Rome has some of the city’s most atmospheric and compelling Christan and ancient sights, in the Colosseum , and, beyond, the churches of San Clemente and San Giovanni in Laterano , among others. It also has some of its most pleasant corners, in particular up on the still green and peaceful Celian Hill .