The ancient castle Castra Martis was built on a steep southern slope above the gorge of the Voynishka River at the end of the 3rd-beginning of the 4th century as part of the Danube Limes. It occupied a key position in guarding the important Roman road from Bononia to Singidunum (Belgrade).
In 408, the Huns under Uldin took control of the site during an attack on the Eastern Roman Empire, apparently by treachery. The fortification was reinforced under Justinian I, but was completely abandoned after the Gothic invasions in 586.
The fortress consisted of two parts: a small square fortification (quadriburgium) measuring 40x40 m, with powerful circular towers with a diameter of 12.5 m in the corners and a castle with the shape of an irregular quadrangle. It covered an area of 1.55 ha. The castle was accessible only from the south, where the gate was situated. Probably at the end of the 4th century, the protection of the gate was reinforced by the construction of another narrower wall.
Foundations of a Roman bath were discovered northwest.The quadriburgium is well-preserved and revealed in its entirety. Its fortress walls, built of stone and three-row brick girders, are 2.2 m thick and 2 m tall, and the southeast tower rises to 16.3 m.
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.