The Burcht van Leiden is an old shell keep in Leiden constructed in the 11th century. It is located at the spot where two tributaries of the Rhine come together, the Leidse Rijn, and another river, now a canal. The structure is on top of a motte, and is today a public park.
From humble beginnings, the hill was raised during various periods of history up to 9 meters above the surrounding landscape in the 11th century. Ada van Holland used the keep as a residence until her father died in 1203 and she was captured by her uncle. In the same year the previous stone building was rebuilt after an attack on the castle, with tuff stone, and after Ada's removal, in 1204 it was attacked again and rebuilt with brick.
Later in the 13th century the building was considered antiquated, since more and more townspeople and houses were built around the base of the hill, making defenses impossible without destroying most of the city. The old 'interior keep' that had been built against the interior walls (a similar ronded keep construction can still be seen in Teylingen) was slowly dismantled and reused for city construction.
As the city of Leiden grew around it in the 13th and 14th centuries, the ruined castle lost its military function. The location became a romantic patriotic symbol after the Siege of Leiden in 1574. In 1651 the city bought the premises to make it into a water tower for public use. A system of waterpipes leading to squares in the city is still intact. In this period a new portal on the keep wall was designed in 1662 with heraldric symbols by Rombout Verhulst, denoting the leading families of the city. There are two other gates to the Burcht, one at the base of the hill with wrought iron heraldric weapons, built in 1653, and one on the south side of the complex, which itself forms a gateway to the park.
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.