The vassal tower-stronghold of Angerja was probably built in the 14th century. The rectangular construction was made of limestone, the walls were two meters thick and it was surrounded by a moat. Some parts of the walls remain to this day and a moat is still visible. It is thought that the stronghold was destroyed during the Livonian war.
Tower-strongholds were built by vassals to protect themselves, mostly against peasant uprisings, especially after the failed St. George's Night uprising of Estonian peasants in 1343.
Reference: 7is7.com
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.