The Church of Saint Catherine is a late Byzantine church in the northwestern corner of the Ano Poli, Thessaloniki. The church dates to the Palaiologan period, but its exact dating and original dedication are unknown. From its interior decoration, which survives in fragments and is dated to ca. 1315, it has been suggested that it was the katholikon of the Monastery of the Almighty.
It was converted to a mosque by Yakup Pasha in the reign of the Ottoman sultan Bayezid II (r. 1481–1512) and named after him Yakup Pasha Mosque. In 1988, it was included among the Paleochristian and Byzantine monuments of Thessaloniki on the list of World Heritage Sites by UNESCO.
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.