Tuida was an early Byzantine and medieval fortress, the remains of which are located on the hill of Hisarlaka in the northeastern part of Sliven, which offers a unique view of the 'Sinite kamani' and the city. It is part of the old mountain fortification system, which played an extremely important role in the defence of the Roman Empire, and later of the early Byzantine Empire, as well as the medieval Bulgarian state. Tuida Castle was originally built in the 4th century AD and was in use until the 13th century.
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.