The Church of St. Nicholas is an Eastern Orthodox Church in the Bulgarian capital of Sofia. The church was built in the 13th century by Kaloyan the sebastocrator as a small family church on today's Kaloyan Street behind the Rila Hotel. It is assumed that the church was erected on the site of the surviving parts of the palace complex that had once stood there which was built during the Roman Empire and accommodated Constantine the Great. Today these parts are preserved and can be seen in the authentic underground walls of the temple.
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.