St. Gereon's Basilica (Basilika Sankt Gereon) was first mentioned in 612. However, the building of the choir gallery, apse, and transepts occurred later, beginning under Archbishop Arnold II von Wied in 1151 and ending in 1227. It is one of twelve great churches in Cologne that were built in the Romanesque style.
St. Gereon has a highly irregular plan, the nave being covered by a decagonal oval dome, 21.0 m long and 16.9 m wide, completed in 1227 on the remains of Roman walls, which are still visible. It is the largest dome built in the West between the erection of the Hagia Sophia in the 6th century and the Duomo of Florence in the 15th century.
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.