Arch of Dolabella
The Arch of Dolabella was an early entrance into the city, built in 10 A.D.! Later it was used to support Nero's aqueduct and it carried water to the Imperial Palace on Palatine Hill.
This church nearby is called Santi Giovanni e Paolo - it's bell tower is built on the ruins of the Temple of Claudius (the white rocks at the base). This cool series of arches was either a medieval buttress from the 2nd century or it was part of a 3rd century street of shops - guess it could have been both!