Aurelian began his new walls to defend Rome in AD 271 and they were completed under Probus in AD 280. They ranged more than twelve miles around Rome, were 60 feet high, were built of brick-faced concrete and had massive towers at intervals of about fifteen yards.
The wall's height was later doubled by emperor Maxentius (AD 306-312). Much of the wall still exists today.
Above is a section near the Porta San Paolo (Porta Ostiensis), below a section, viewed from inside the city, close to the Castrensian Amphitheatre.