The Vastseliina Castle was a castle of the Livonian Order, Bishopric of Dorpat. It was constructed in 1342 by the Landmeister Burkhard von Dreileben as part of the border fortifications of Old Livonia against Novgorod, Pskov and later Moscow. In the Middle Ages, Vastseliina Castle was well known in the Catholic world as a popular destination for pilgrims. They worshipped the holy cross in the castle chapel and a visit to the chapel gave them sanctification for 40 days – it was first validated by Pope Innocentius VI in 1354.
The castle met its end during the Great Northern War when it was demolished by Russian troops (1702). Today the ruins are open to the public.
Reference: Wikipedia, VisitEstonia
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.