Greifenstein castle overlooks the Danube river. Approximately opposite is Burg Kreuzenstein, on the north shore of the Danube. The castle is thought to be built around the 11th century, and was first mentioned in 1135. However, the owners of the castle changed owners frequently during its service from the 11th century to 1918. It began life playing a significant role in the defense system along the Danube.
More recently, in the 16th century, the castle served primarily as a notorious prison of the ecclesiastical court. Throughout history, the castle was repeatedly invaded and damaged, but repaired, again and again. It was inhabited until about 1770, when it was abandoned and fell into disrepair. Until 1803, it belonged to the Bishops of Passau.
Johann I von Liechtenstein refurbished it in 1807–08 in the Romantic style but at the end of the 19th century, the castle fell again and was sold in 1918, coming into private ownership.
Today The castle has remained in private ownership and is presumed to be on sale.
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.