Answer:
They built a wall around the city to keep out invaders
Step-by-step explanation:
During the reign of Aurelian and Probus, the Aurelian Walls were built around Rome. They built this wall because the Germanic tribes were becoming a threat to Rome so the wall was a way to protect the city. The Wall had 19 km long and 6 to 8 meters high.
In the end, the Aurelian Wall was a good idea because it took about 100 years so the barbarians reached Rome, that invaded the city in 410 marking the end of the Roman Empire.