The aqueduct of Fréjus was built in the middle of the first century after the ramparts were in place. It functioned for 450 years until the 5th century. It is 42 km long, with a difference in altitude between the highest spring of Neïssoun and the castellum aquae in the city of 481 m. The aqueduct runs mostly in a covered conduit for 36.4 km and for 1.8 km on bridges and 500 m on walls. Large parts of the aqueduct are still well preserved.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.