Ehrenfels Castle was (re-)built about 1212 at the behest of the Archbishop of Mainz as a defensive work against the constant attacks by Elector Palatine Henry V, who, as Imperial vicar of Franconia, strived to cut down the archbishop's reach. Mainz staffed the castle with Burgmannen and erected a customs post controlling the shipping on the Rhine, supplemented by the Mouse Tower below at the river. Heavily damaged in the course of the Thirty Years' War, the castle was finally devastated by French troops under the command of Lieutenant General Nicolas Chalon du Bléduring the 1689 Siege of Mainz.
The ruin can be reached from Rüdesheim via a hiking trail through the vineyards. The interior however can only be visited in guided tours by prior appointment.
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.