Santo Stefano d'Aveto Castle controlled the roads from the Apennines to Emilia. It was probably built int the 12th century, first time documented in 1164.
The castle that presents itself now is an interesting pentagonal structure with ramparts on four of the five perimetral corners. In ancient times the arms square could be reached, an area onto which the different rooms on the ground floor open, using drawbridges that were easy to lift in case of attacks.
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.