The Walls of Constantinople were initially constructed by Constantine the Great to enclose and safeguard the emerging city from potential attacks by both land and sea. As the city expanded, the renowned double line of the Theodosian Walls was built in the 5th century. Although the remaining sections of the walls were less intricate, they were incredibly challenging to breach for any medieval attacker, especially when appropriately manned.

These walls played a critical role in defending the city and preserving the Byzantine Empire during sieges by various groups such as the Avar-Sassanian coalition, Arabs, Rus', and Bulgars. Despite the introduction of gunpowder siege cannons, which made the fortifications more vulnerable, the technology was not advanced enough to capture the city independently, and the walls could be repaired during reloading. Nonetheless, on 29 May 1453 CE, after a two-month siege, the city fell to the Ottoman forces due to their overwhelming numbers.

circa 324-336 CE

Constantinian Walls
During the years 324-336 CE, emperor Constantine reconstructed the city and built a new wall approximately 2.8 kilometers (15 stadia) west of the Severan wall to protect the new city. The fortification consisted of a single wall with tower reinforcements at regular intervals, which was initiated in 324 CE and completed under Constantius II (reigned 337–361 CE). However, only the rough course of the wall is known today.

By the early 5th century CE, Constantinople had expanded beyond the Constantinian Wall into the Exokionion or Exakionion extramural area. Despite being replaced by the Theodosian Walls as the primary defense of the city, the Constantinian Wall survived for most of the Byzantine period. Although some parts of the wall may have remained intact until the mid ninth century CE, it collapsed during an earthquake in 867 CE, as reported by the 11th-century CE historian Kedrenos. Later, only fragments of the wall are believed to have remained as Van Millingen suggests that some sections survived until the early nineteenth century CE in the region of the İsakapı.

circa 439 CE

Theodosian Walls
The double Theodosian Walls were built by Emperor Theodosius II, and they are located about 2 kilometers west of the old Constantinian Wall. They were named after the emperor who ordered their construction, and they were built in two phases. The first phase was carried out by Anthemius, the praetorian prefect of the East, during Theodosius' minority and was completed in 413 CE according to a law in the Codex Theodosianus. However, an inscription found in 1993 CE suggests that the construction lasted for nine years, which means that it began around 404/405 CE during the reign of Emperor Arcadius. This first phase included a single curtain wall with towers that now makes up the inner circuit of the Theodosian Walls.

The Theodosian walls were badly damaged in two earthquakes that occurred on 25 September 437 CE and 6 November 447 CE. The second earthquake was especially powerful and destroyed many parts of the wall, including 57 towers. The damage was compounded by subsequent earthquakes, including a major one in January 448 CE. In light of the imminent threat posed by Attila the Hun, Theodosius II instructed the praetorian prefect Constantine to oversee the repairs. To expedite the work, the city's "Circus factions" were employed. The walls were restored in a record time of 60 days, as recorded by Byzantine chroniclers and three inscriptions found at the site. Most scholars believe that this is when the second, outer wall was added, along with a wide moat in front of the walls. However, this interpretation is uncertain, and some argue that the outer wall may have been part of the original fortification plan.

The walls suffered damage from earthquakes and floods of the Lycus river throughout their history, and repairs were carried out multiple times. There are numerous inscriptions commemorating the emperors or their appointees who oversaw these repairs. The Domestic of the Walls or the Count of the Walls was responsible for these repairs and enlisted the help of the city's inhabitants. After the Latin conquest of 1204 CE, the walls gradually fell into disrepair, and the revived Byzantine state after 1261 CE lacked the necessary resources to maintain them, except in times of immediate danger.