The Temple of Augustus (exact location unknown) was built by Tiberius and possibly his mother Livia (Augustus' wife) in honor of Augustus. Tiberius might not have finished it, or might not have dedicated it, in which case it was finished and/or dedicated by Caligula. The large structure behind the Temple of Castor and Pollux was wrongly identified as this Temple of Augustus in the late nineteenth Century, so the plaque you see there is wrong. And it is also wrongly placed on the other side of the Temple of Castor and Pollux because a ground level concrete base that supported a building was found there. Odds are it was located a little farther down the Vicus Tuscus (right side) and fronting the Vicus but now any remains are buried beneath the modern city level. The hint for that location is that Caligula's rooftop bridge crossed-over from the Palatine Hill *to* the Temple of Augustus according to ancient historians.
After you pass the side of the Basilica Julia and the trees you will likely see the queue for the bathroom / W.C. In front of this WC are two brick pier ruins. There was a side street that connected the Vicus Tuscus to the Vicus Jugarius (the parallel street on the other side of the Basilica Julia that I mentioned earlier). These two brick piers were once clad in marble and formed an arch across the beginning of this street, giving this short street a nice monumental entrance. Look closely especially at the left pier and you can see the small bronze(?) nails that held the marble slabs in place also small bits of marble remains.
In about 2005 I took a private tour at the group rate (the others didn't make it) with Tom R. co-founder of the American Institute of Roman Culture which had just completed an excavation of this area www.romanculture.org/PROJECTS/Roman_Forum/Dig_Official/progress.html (Trench D) he is also the founder of Scala Reale which is now Context Rome. I'm relying on memory and do not wish to misquote Tom so I'm not going to use his last name so it doesn't come up in a Google search or those 'spider' thingies :-) . On top of the left pier a baby or small child's tomb was found, I believe it was Medieval. I assume it was like the tombs cut into the brick front of the Curia Julia and then bricked-over?
The right pier had an original doorway built into it, the bottom half is still below ground level. They think that this arch was going to block access to one of the older shops that lined this street so the shop was accommodated with a doorway.
This street was called the Vicus Unguentarius or the 'Street of the Perfume Sellers'. (So keep that in mind as you pass between the arch's two brick piers and into the odorous WC :-) . The Romans loved a good scent and these were high-end shops.
Later in time (medieval era?) this arch was likely turned into a dwelling and/or a shop with a new floor put in. On each pier you can see holes for wooden beams that made the interior of the arch into two floors. Possibly that child's tomb was built in his parent's dwelling?
Ok now, leave the WC and walk 45° to the right and over to that fence. That is still the Vicus Tuscus leading to the Circus Maximus. On the Palatine Hill side of that street is the 'HORREA AGRIPPIANA' built at the end of the first Century BC by Augustus' son-in-law and closest friend Marcus Agrippa (an altar was discovered there with an inscription recording the erection of the 'Statue of the Genius Horreorum Agrippianorum'). It was a large grain warehouse built around three courtyards each with three stories of rooms. So you're looking at the first courtyard and where the church (San Teodoro) is, is the second courtyard and the third courtyard is beyond that. High on the walls are holes, horizontal and diagonal cuttings these were later adaptations of this building some as recent as the 1800's.Next: #18.3: Domitian's Hall