The grassy/tree area, with the concrete arches at street level, is an artificial platform (175x205m)
built for the 'Temple of Deified Claudius.'  The arches were built-up there to make it level.
It extended back to the belltower at the left-center of the photo.

Nero's mother (Claudius' niece and widow!) started it in 54 AD, but in 59 AD work was stopped when
Nero murdered his mother.  Emperor Vespasian completed it in the early 70s AD.

Thanks to Walter ("Paradise Lost") of Frommer's Forum for the information!