Next to the area with the Julia Basilica, on top of a podium you can see three beautiful, grooved columns with leaf capitals holding up a piece of a lintel. They have almost become the symbol of the entire archaeological area and are the most important part of a very ancient temple called the Temple of Castor and Pollux, which was dedicated to the twins born from the love between Leda and Jupiter, who seduced Leda in the guise of a swan. The temple was home to the office that verified weights and measures, and was rebuilt in the 6th year AD by Emperor Tiberius, as the columns date back to this era. I'd also like to point out a beautiful fountain in the form of a temple nearby here.
Continuing on, you'll see the remains of the church of Santa Maria Antiqua that were recently restored and have been reopened to the public. This building is quite important because it was the first place of Christian worship that was consecrated in the Roman Forum; some of its original imperial palace rooms were re-adapted to do so, and the palace dates back to the sixth century. You can admire beautiful early Christian frescoes here that are part of an exceptional complex of paintings that come from the sixth to the ninth century. You will be amazed by a curious Crucifixion where Christ is not presented naked, but with a long gown down to his feet.
You've basically reached the center of the forum, in the sacred area of goddess Vesta, the protector of private homes. The goddess' small circular temple was surrounded by a colonnade of which only a few parts remain. Inside, a fire was carefully guarded that could never go out. The House of the Vestal Virgins is near the Temple of Vesta: it was made of bricks with a central courtyard overlooking the houses of the priestesses and the warehouses for the precious pledges and offerings that were given to the temple.
The Temple of Antoninus and Faustina was also saved thanks to its transformation into a Christian church dedicated to St. Lawrence: it was erected in 141 and was only dedicated to Faustina at first, who was divinized after her death, then also to her husband, Emperor Antoninus Pius. You can still admire its cipollino marble columns and the distinctively elegant frieze that decorates the cell.
Right next to it a necropolis was discovered with grave tombs dating back to about the ninth century BC: the oldest settlement in the area!
FUN FACT: if you look at the bases of the columns of the Temple of Antoninus and Faustina, you'll notice strange marks and grooves: they were made in an attempt to knock it down by pulling on the columns with pairs of oxen. But fortunately the ancient, 17-meter tall columns resisted!