San Esteban de Deyo, also called the Castillo de Monjardín, is a ruined castle on a hill overlooking Villamayor de Monjardín in Navarre. The castle lies at an elevation of 890 metres. The castle has a Roman foundation, but was repeatedly rebuilt over the centuries. It was one of the last fortresses of the Banu Qasi, the local Muslim dynasty, before it was taken by King Sancho I of Navarre in 914. According to the Historia Caroli Magni et Rotholandi, the castle was actually taken by Charlemagne from a Navarrese prince named Furré. Only the walls of the castle stand today. Many of the stones were taken to build a Baroque hermitage, the Ermita de San Esteban, in the 17th century.
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.