One of the most important churches of Mystras is the Cathedral of Agios Demetrios, built in 1280- 1292. It is the oldest church in the city.
This church has a mixed architectural style: on the one hand it has a three-aisled basilica with a narthex and a bell tower, built in the 13th century on the ground floor, and on the other hand, the upper floor is a cross-in-square church, added in the first half of the 15th century. Its interior is decorated with wall paintings in many different styles. This is where the last Byzantine emperor, Constantinos Paleologos, was crowned in 1449.
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.