The Order of the Brothers of the Sword began building Sigulda castle in 1207. Initially, it was a castellum-type fort; following the defeat of the Brothers of the Sword in 1236, the Livonian Order modified it. During the Great Northern War, the castle was destroyed and never restored again. The south-west section of the convent building with Gothic window lintels and the main gate tower have survived to the present day.
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.