Shiryōkaku (四稜郭) (literally, 'four-point fort') is a fort in the city of Hakodate. It was constructed in April 1869, during the Battle of Hakodate, three kilometres to the northeast of Goryōkaku by two hundred soldiers of the former Tokugawa shogunate and a hundred local villagers, likely under the direction of Ōtori Keisuke.
The fort covers an area of 21,500 m2, stretching approximately a hundred metres east to west and seventy metres north to south; the earthworks rise to a height of 3 m with a width of 5.4 m; they are surrounded by a dry moat 0.9 m deep and 2.7 m wide; the entrance is to the southwest.
Shiryōkaku fell to government forces within a few hours on 11 May 1869.
In 1934 the area was designated an Historic Site. Repairs were carried out from 1970-2 and again in 1990.
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.