The basilica is called “outside the walls” because it was outside the ancient city of Rome, constructed between 271 and 275 AD by emperors Aurellius and Probus. The walls enclosed the seven hills, plus the fields of Mars: Paul was killed outside the walls to dishonor him.
Emperor Constantine constructed a church there, on the site of an earlier, 1st-century memorial. Some archeologists were skeptical of this; you can still see words like “allegedly” and “according to legend,” despite Benedictine assertions they found the insciption, “Paolo, Apostolo, Mart[yri]” when the basilica was rebuilt in the 1820s. In 2006, excavating beneath the basilica was found the sarcophagus.
This is the body of the St. Paul.
Oops, I meant to state that the WALLS were built by Aurellius and Probus!
How was this possible, since the walls you refer to were built over 200 years after his death?