This shot is obviously (well, perhaps not obviously) taken from the Colosseum and past the obligatory crowd.
The ruins in the background are the remains of the temple to Venus and Roma. It was essentially a dual temple; two temples in the one complex back to back. As I understand it, what we're looking at is the apse of the temple to Roma where her statue once stood. The complex was the brainchild of the Emperor Hadrian. (That is, Caesar Trajanus Hadrianus, or Publio Elio Traiano Adriano, who ruled from 11-Aug-117 to 10-Jul-138.) It was constructed between 121 and 141 CE. (Thus Hadrian didn't live to see its completion, having died in 138. However he inaugurated the temple in 135. It was completed by his little remembered but apparently quite capable son Antoninus Pius (Antonino Pio), who ruled from 11-Jul-138 to 7-Mar-161.)
We didn't get much of a chance to see it. However even if we had, unfortunately I'm not sure that we would have had as great an appreciation for what it once was as it would deserve. The temple complex was reputed to be the largest ever built in Rome. It was probably the basis for the mental image that people have of a Roman temple; large, surrounded by colonnades and featuring an abundance of gold and marble.
To give some idea of its size, you can see in the background the bell tower of the Church of Santa Francesca Romana. Apparently in its prime, the complex was that high.
Sadly, as was the case with many buildings in Rome it was eventually looted to provide building materials for other buildings.
Oh, to have the ability to transport myself back to say 150 CE for a few hours, and stand on this spot with a full frame DSLR, some bitchingly good glass and a few empty high-capacity memory cards.
This is another example of where I made duplicates of the image, adjusted the exposure in Camera Raw and then merged them to HDR to try to pick up at least some of the detail of the inside of the wall.