Ok now still standing next to the Arch of Augustus let's view some hard to see sites in that fenced-off area on the right (south).
Look through that arched door opening in a wall to the right of that small modern looking building. You can't see much but it's the 'CHURCH OF SANTA MARIA ANTIQUA', photographed here. It was built in the fifth Century in a space that was once a monumental approach up to the Palatine Hill Palace. In 847 an earthquake undermined the structures above it and it was pratically buried in the rubble and abandoned. In 1084 the Normans sacked Rome and set a fire that collapsed more structures which completely buried it. In 1902 this church was excavated. It has a few frescos dating to the seventh and eighth Century which have recently been restored and also seventh and eighth Century Church Council decisions were recorded on walls. In Medieval times it was believed that this abandoned Church was haunted. It's been planned to be open to the public (reservation only) already but as of now it's still not.
Now that modern looking building I mentioned earlier, it's to protect this site. 'ORATORY OF THE FORTY MARTYRS' which was part of that church. In 303 AD during Diocletian's Christian Persecution 40 Christian soldiers were forced into an icy lake or pool in Sebaste, Armenia where they froze to death. And this about eighth Century Oratory was dedicated to them although it was originally Pagan built between 117-138 AD for something else. Inside are eighth and ninth Century frescos showing their torture and martyrdom.
Notice the side of that building, there is a marble structure like a doorway (two columns supporting a peaked roof) that is the reconstructed 'AEDICULA OF JUTURNA'. This Aedicula (shrine) is dedicated to Juturna who was a nymph and goddess of springs and fountains. This shrine dates to the second Century AD which replaced an earlier same shrine. Within the shrine there was a marble statue of Juturna and inscribed on the architrave "Here was the true cult site of the Nymph". In front of the shrine is an altar and a well-head (plaster casts of the originals). The altar has two figures on it 'Turnus and Juturna' and the well-head's rim is inscribed with the name of the man who dedicated it, 'M. Barbatius Pollo' who was the 'Curule Aedile' at the end of the first Century BC.
Now look between you and this shrine, see a lone column and a square basin area made by stone blocks. That is the 'SPRING OF JUTURNA' (Lacus Juturnae) and tied-in to the Temple of Castor and Pollux as it was here that Castor and Pollux magically appeared and were seen watering their horses after that 496 BC battle. This was a real spring, by the way, and was used well before the founding of Rome right up until first aqueduct was built in 312 BC and later when Rome Fell and the aqueducts were destroyed it was again used into early Medieval Times (eighth Century AD water jugs were found here).
The fountain's basin is a little over 5 m x 5 m and 2 m deep. In the center of the basin was a square marble platform (1.8 m high, 3 m long, 2 m wide) [``=``] with statues of Castor and Pollux. The oldest part of the fountain dates to about 164 BC. In front of the fountain stood a round marble well (Augustus era) and an altar (about 200 AD). Also the Twins made another magical appearance at the fountain in 168 BC after another Roman military victory :-) .
And if you look at the ruins to the right of this site you can see the remains of a ramp going up. This ramp connected the Roman Forum with the Via Nova which is an elevated street on the lower slope of the Palatine Hill which ran behind the House of the Vestal Virgins.
Next: #22: Regia