The length of the historical walls surrounding Istanbul is twenty-two kilometers. There are ninety bastions on the land side walls.
The ramparts surrounding Istanbul were built the first time in the 5th century, and were renewed four times due to demolition and re-building. During the era of Theodosius II (408-450) the ramparts of Istanbul stretched from Sarayburnu to Ayvansaray along the Golden Horn, to Yedikule along the Marmara, from Yedikule to Topkapı, and from Topkapı to Ayvansaray. The ramparts are a total of 22 km long; on the Golden Horn they are 5.5 km long. The ramparts are 7.5 km long on land and 9 km. long on the Marmara coastline. The land walls are composed of three sections, the moat, the external ramparts, and the internal ramparts. The moats are today used as agriculture fields. With a projection of 10 meters from the rampart walls, the towers have a square plan and are generally 25 meters high. They have windows, doors and vaults. The doors and stairs between the internal ramparts and the external ramparts are still visible. The internal ramparts and bastions are made of sandstone and bricks. The external walls, however, are mainly box walls or casemate walls. All these moats, bastions, and external and internal ramparts are 70 meters wide. They include corridors and small holes. On the Marmara and Golden Horn walls there are no moats or external ramparts. The width of these ramparts is 5 meters, and the height is 15m. The bastions are 20 meters high, On the Marmara side, there are 103 bastions, and 94 on the Golden Horn side. Yedikule, Sulukule, Anemas, İsaakios, Mermerkule, and Arapkule are historically important. Most of them were used as dungeons or prisons, and some also served as mints.
There are primary and secondary gates along the ramparts of Istanbul. The gates are under a 5 meter wide arch in the walls. These gates had marble-coated internal sides and wooden external cases. There is distance of 25 meters between the external gate and the internal gate. There are 36 small and large gates connecting the interior of the city walls to the outside.
The primary gates from the Topkapı Palace gate and to the Golden Horn, on to Yedikule and back to Topkapı Palace are as follows:
The Golden (Silver) Gate
The Golden Gate was used during the Byzantine era only for special ceremonies.
The Belgrade Gate
This gate in Yedikule is the second largest military gate and dates back to the 5th century. It was used for military exercises.
The Silivri Gate
One of the country gates is today known as Silivrikapi. During a restoration in 1987, a tomb was discovered.It consisted of two sections housing five tombs from earlier ages.
The Kalagru Gate
This was a small military gate between the Pege and Rhesium gates used during the Byzantine era.
The Mevlana Gate
During the last periods of the Byzantine Empire, a Russian community that had settled in Eyüp in the 9th century was allowed to enter the city through this gate. Thus it was also known as the “Russian Gate”. During the Ottoman Empire, the gate came to be known as the “Mevlevihane Gate” because a Mevlevihane was located in the neighborhood.
The Topkapı (Cannon Gate)
During the conquest, Fatih Sultan Mehmet established his headquarters at this point and set up the largest cannons here and the cannons were fired at this point.
The Sulukule Gate
Since it is located above the Lykos Brook, this was named Sulukule (the Water Tower). Following the conquest, the gypsies settled in this area. They are now being relocated.
The Edirne Gate
It is said that this was the first gate to be opened during the conquest. The Byzantine emperors who went voyages, merchants from Rumelia, and foreign ambassadors would enter the city through this gate.
The Eğri Kapı (Crooked Gate)
It is said that artisans making shoes and boots for the army during the Byzantine era were located in this area.
The Blakhernai Gate
This was one of the civilian gates giving access to the Blakhernai Palace and its surroundings. It includes four towers, built by order of Byzantine emperors, and three other towers built by order of Fatih Sultan Mehmet.
It is made of four towers, built under the order of Byzantine emperors, and 3 other towers built under the order of Fatih Sultan Mehmet.
Genç Osman Tower
This tower is named after Genç Osman, who was killed on the second floor of this tower.
The Ammunition Tower
As may be inferred from its name, it was used as an ammunitions warehouse during the time it belonged to the state.
The Tower of Ahmet, III
Ahmet III contributed tothe building and restoration of the tower to a great extent and ensured that the floors were supported with beams.
The Treasury Tower
This was once a place where the treasury owned by the State would be kept.
The Dungeon Tower
Also known as the Tower of Tablets, it is one of the towers used as a dungeon.
The Cannon Tower
This was also used as a prison.
The Flag Tower
Since this was the place where the flag flew, the janissary troops would guard this area. It still retains its robust structure even today.