Viljandi castle was one of the strongest castles in Livonia. The construction was started 1224 under Teutonic Order in place of a former hillfort. The crusaders of Sword Brethren conquered the hill fort at the place of later main castle in 1223. A year later, construction of stone fortifications started. Viljandi was chosen as the high seat of the order.
The convent house, a typical form of castle of Teutonic Knights, was erected late 13th – early 14th century. In the following centuries the castle was extended and fortified further. It was badly damaged in the Polish-Swedish wars in early 17th century and not repaired any more. In 18th century, the ruins were used for quarrying stones for construction work in Viljandi.
The first excavations in the castle were performed in 1878–1879. In recent decades, these have turned to almost yearly events. Currently the ruins form a popular resort area just outside of central Viljandi. An open-air stage is located in the former central courtyard.
Reference: Wikipedia
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.