Urkulu (1,419 m) is a mountain in the Basque Country straddling the border between France and Spain. The main feature of the mountain is the remains of a Roman tower topping the summit, erected in the 1st century BC. to commemorate the recent conquest of Aquitaine.
With the summit providing excellent views over the northern and southern slopes alike, it was used as a watchtower in medieval times.
The tower has a truncated-conical shape, measures 19.5 meters in diameter at the base and is 3.6 meters high. Its original height should have been 4.5 meters. The thickness of the walls is 2.6 meters and its interior is filled with the remains of the original stonework.
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.