The Skopje Fortress, commonly referred to as Kale, is situated on the highest point in the city overlooking the Vardar River. The fortress is depicted on the coat of arms of Skopje, which in turn is incorporated in the city's flag.
The first fortress was built in 6th century AD on a land that was inhabited during the Neolithic and Bronze Ages (roughly 4000 BC onwards). It was constructed with yellow limestone and travertine, along with fragments of Latin inscriptions. Material for the fortress originated from the Roman city of Skupi, which was completely destroyed by an earthquake in 518.
The fortress is thought to have been built during the rule of emperor Justinian I and constructed further during the 10th and 11th centuries over the remains of emperor Justinian's Byzantine fortress which may have been destroyed due to a number of wars and battles in the region. The city was capital of the First Bulgarian Empire between 992 and 1015, and was center of the uprising of the Bulgarian Empire against the Byzantine Empire under the rule of Peter Delyan. Not much is known about the Medieval fortress apart from a few documents which outline minor characteristics in the fortress' appearance.
In 1346 at the Skopje Fortress, Stefan Dušan adopted the title of Emperor at his coronation and he had transferred the capital of Serbian Empire to Skopje.
The fortress was partially destroyed yet again by an earthquake in 1963 but was not reconstructed until recently.
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.