According to the diary, what did constantinople depend on for its defense?

Constantinople was founded on the former greek colony of Byzantium by the new emperor Constantine on 330 A.D. It was considered later on as the richest city in Europe during the 3rd to 14th century. It depends on land walls for its defenses. These walls surround the cities on each side, protecting it from both land and naval attacks. The most famous was the Theodosian walls which was built during the 5th century.  These walls were considered as one of the most elaborate system ever created.