The remains you see here were part of the Basilica of St. Euphemius built in 1031 by Litigerius, bishop of Como. It was demolished by the city of Como in 1169 (historical sources document the deed of foundation). Archaeological excavation has revealed the existence of an earlier religious building, erected during the Early Christian period or the Early Middle Ages. The earliest ruins date back to Roman times.
The Romanesque building was a large three-aisled basilica (22×62 m); the aisles, separated by octagonal masonry columns, terminated in three semicircular apses facing east. A central staircase of 9 steps allowed visitors to ascend to the raised presbytery while in the wings two smaller staircases led down to the crypt. Given the composition of the terrain (for the most part bedrock), the right apse was placed at the same level as the aisles while the left apse was built on a somewhat higher level.
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.