Castra ad Montanesium is a ruined Roman fortress in the town of Montana, Bulgaria. It was built as the town grew to greater importance as a Roman settlement. Near the entrance of fortress are the ruins of the basilica. The built tourist path passes along it and leads to the fortress walls, behind which there is another church, though it is smaller.
The fortress was constructed in the first century AD to give a defensive buff to the Roman town of Montanesium. During the reign of Constantine the Great (306-337) an early Christian basilica was constructed adjacent to the complex.
Between 440 and 490, the northwest of modern Bulgaria was devastated by the raids of the Huns, under Attila, and the Goths. Later raids by the Slavs and Avars between 500 and 560 resulted in the destruction of the fortress down to its foundations, along with most of Montanesium.
In the early 2010s, an initiative was undertaken to restore parts of the basilica and fortress, as a part of the second stage of the EU's Regional development program.
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.