The Yellow Fortress or Yellow Bastion is a cannon fortress at the entrance of the 'Walled City of Vratnik'. It was built between 1727 and 1739 in area called Jekovac, close to the Jajce Barracks and the Jekovac water reservoir. It served as one of the defense points against the Austro-Hungarian troops in 1878. The fortress was damaged and rebuilt several times. The most recent renovation took place in 1998.
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.