The Aurelian Wall is by no means the first wall around Rome. For a while the Empire thought its borders were secure enough effectively to be Rome’s walls, but that changed in the 3rd century AD, when tensions and insecurity permeated Roman society. For the first time in 700 years, Rome needed a new wall. The Emperor Aurelian caused the wall to be built between 270 and 275 AD, which seems to me a phenomenally short time, given the size and scope of the thing -- 12.5 miles long and fortified every 100 feet with a tower. Building was, apparently, a private-public partnership that involved the army and private builders.
There are 40 major gates and several minor ones and originally, as can be seen in the photo, the walls were only about half their finished height. Building fast, the walls made use of anything to hand, incorporating a small colosseum that stood here, near the church of Santa Croce in Gerusalemme. That church was built much later than the wall and was founded by Helena, mother of Constantine. It was Constantine who founded San Giovanni in Laterano. Santa Croce has a good collection of relics, including not only three fragments of the True Cross, but also a nail, two thorns (from the crown of the same) and a finger bone said to have belonged to St Thomas.