The structures built to protect the ancient city of Rome were primarily the massive walls known as the Aurelian Walls. Constructed in the 3rd century AD, they encompassed the city's boundaries and served as a significant defensive measure against invasions. The walls were built using various materials, including brick and concrete, and featured numerous gates for entry and exit. Additionally, Rome had fortifications such as watchtowers and bastions at strategic locations. The Aurelian Walls played a crucial role in safeguarding the city throughout its history, particularly during times of conflict.