The Heidenmauer ('heathen wall') is a circular rampart or ringwork, two and a half kilometres long, which was built by the Celts around 500 BC. The wooden elements of the wall have disappeared over the course of time by rotting away, but the stones have survived. Copious numbers of pottery finds have enabled a very precise dating. Almost all the containers are hand-made, only a few show traces of having been turned; this technology first appeared after ch 500 BC. in the La Tène period. Other finds included iron, long knives as well as querns, pyramidal stones that were stuck point-down in the ground in order to provide the base for the milling of corn. In addition there is also evidence of milk production and iron smelting.
In the 4th century A. D. a small part of the circular rampart as well as the Kriemhildenstuhl below was used by the Romans as a quarry.
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.