The Theodosian Walls are among the most impressive monuments of Late Antiquity. These land walls protected Constantinople for centuries while the other great cities like Rome, Antioch or Alexandria were sacked. The Theodosian Walls consists of an inner wall, an outer wall with an inner terrace, and a wide moat, with a second outer wall in front of it. Attackers first had to cross the moat and its wall, while being fired upon from both the outer and inner walls – the towers of the inner walls were equipped with ballistae and catapults. Even if some attackers were able to scale the outer wall, they would then be trapped on the inner terrace, facing a taller wall reinforced by huge towers.  As Constantinople lies on triangular peninsula surrounded by the sea on two sides, its land walls were frequently the main defenses required. The triple line of defense of the Theodosian Walls, then, helped make Constantinople virtually invulnerable for centuries. 

The Theodosian Walls are now around 5.5 kilometers long, but once were longer when they continued through the Blachernai. The walls and towers were constructed with limestone blocks divided at intervals by layers of red brick with a core of mortared rubble. The inner wall is reinforced by 96 polygonal and rectangular towers. The height of the inner walls is around 10 meters above the outer terrace and as much as 13 meters above the ground within the city. The wall is around 4.5 meters thick, with tower roughly 75 meters apart with a height around 19 meters. The inner terrace is around 16-21 meters wide. The outer wall also has towers at regular intervals and is around 9 meters above the outer terrace, with walls as much as 2 meters thick. This wall, reinforced by relieving arches supporting the walkway, had an addition 92 towers. The outer terrace is around 20 meters. The moat is around 20 meters wide, with a maximum depth of 7 meters. In some towers of the outer wall, there are posterns giving access to the outer terrace. The Theodosian Walls have six main gates and several secondary posterns. 

The first walls of Constantinople were built by Constantine I following his victory at the Battle of Chrysopolis in 324. While these walls were able to defense the city from the Goths following their victory at the Battle of Adrianople in 378, it was becoming clearer that Constantine’s city walls were inadequate. Furthermore the population had grown to the extent that new development of the city began to take place outside of the walls. Early in the reign of Theodosius II (408–50) the need for better defenses at Constantinople became abundantly evident following the Sack of Rome by the Goths in 410. 

The walls were completed in two phases during the reign of Theodosius II (408–50). The first construction phase, ordered by the regent for the young Theodosius II, the Praetorian Prefect Anthemius, was completed around 413. It is possible, though, that a new line of walls was first conceived by Theodosius I, as the construction of the Porta Aurea (“Golden Gate”) was completed by the time of the emperor’s victory procession in 391. It consisted of a single wall and a series of towers located approximately 1.5 kilometers west of the Walls of Constantine and stretched around 6.5 kilometers from the Golden Horn to the Propontis (Sea of Marmara). The prefect Cyrus Panopolites built or extended walls along the Marmara and the Golden Horn in 439. It seems that this wall was extensively damaged as a result of severe earthquakes of 437 and 447. As a result, repairs were quickly made under the direction of the Praetorian Prefect Constantine. The work was accomplished with the help of the urban population in two months, partly because the Huns occupied the Balkans during the period and were a direct threat to Constantinople itself. The scale and speed of this remarkable feat is commemorated by Latin and Greek inscriptions at the Rhegium Gate (Turkish Mevlevihane Kapısı). This was not merely a reconstruction of the walls, but rather consisted of extensive enlarging and reinforcing of the walls, by adding an outer wall and a moat. This would result in Constantinople having the most elaborate and complex urban fortifications in the ancient world. These walls, though, did have their weak points. The outer wall seems to have ended a short distance north of the Charisios Gate, while the moat seems to end just south of the Charisios Gate. The most vulnerable section of the walls, which was known as the Mesoteichion, seems to have been around the valley of the Lykos River. The addition walls, later constructed around Blachernai, were also a vulnerable section of the land walls. 
Starting in the late 6th century, the Byzantine Empire entered a period of crisis which would soon put the city’s defenses to the test. Constantinople faced its first major threat during the Avar and Persian siege in 626. The Avars, who bombarded the Theodosian Walls with siege engines, were unsuccessful in their attacks, partly due to their inability to coordinate with the Persians and their fleet. The Arabs, who had successfully conquered the empires’ eastern provinces, besieged Constantinople in 674 and 717-718. While these were a much greater threat as the Arabs had both an army and a fleet, they were ultimately unsuccessful as well. Following the second siege of the city, the Byzantine Empire slowly began to recover, resulting in Constantinople no longer being directly threatened. However another army appeared before the walls of Constantinople in 813. The Bulgarian Khan Krum (c. 803-814), who had just crushed the Byzantines in battle, killing Emperor Nikephoros I (802-811), brought his armies before the Land Walls but found that there was very little that he could do. 

The land walls were repeatedly damaged by earthquakes, and thus required repairs. These repairs are often commemorated by various inscriptions on the Theodosian Walls. One such inscription, located at the Rhegium Gate, records repairs made Spatharios Narse, on initiative of Justin II (565-578) and his wife Sophia. Furthermore, additional walls were later constructed around Blachernai in order to bring forward the walls from the original line of the Theodosian Wall. The first addition to the land walls was made during the reign of Heraclius (610-641) following the Siege of Constantinople by the Avars and Persians in 626. Justinian II (685-695) and Anastasius II (713-715) made repairs of the walls in preparation for an attack by the Arabs. Following an earthquake in 740, Leo III (717-741) and his son, Constantine V (741-775) repaired the walls, as indicated by an inscription on Tower 25. A series of earthquakes during the reign of Basil II (976-1025) required repairs, which are commemorated by inscriptions on Tower 1, Tower 36 and Tower 50 at the Rhegium Gate. During the reign of Manuel I Komnenos (1143-1180) a new wall was built to better secure the Palace of Blachernai, as it was the primary imperial residence at the time. In addition, the walls were extensively repaired during his reign. The walls were neglected following the Latin occupation of Constantinople in 1204, and while the Palaiologan dynasty restored Byzantine rule in 1261, they generally lacked the financial resources to maintain the walls on a regular basis. Nonetheless both Michael VIII Palaiologos 1259-1282) and his son Andronikos II (1282-1328) had repairs made to the walls. The brief siege of Constantinople in 1422 by Sultan Murad II (1421-51) revealed the imminent threat of the Ottomans. Accordingly, John VIII (1425-48) had the walls extensively repaired, as multiple inscriptions, including some recorded but now lost, record. 

The Theodosian Walls played a prominent role during the Siege of Constantinople in 1453 by Mehmet II (1451-1481). The accounts of the siege and subsequent sack of Constantinople vary quite widely, making it difficult to precisely determine its exact details. The Ottoman army, which began to besiege Constantinople in early April 1453, bombarded various sections of the walls with large canons cast by Urban, an engineer possibly of Hungarian origin. While significantly damaging the walls in certain sections, the defenders of the city were able to repeatedly fight off enemy advances. The Ottomans also failed in their attempt to attack the walls with a siege tower as well as multiple attempts at mines under the walls. During the siege, the land walls were defended by the Genoese Giovanni Giustiniani Longo, who had brought 700 soldiers and offered his services to Constantine XI in early 1453. He withdrew from the walls when he was wounded on May 29 leading many Genoese soldiers to panic around the Pempton Gate. The ensuing chaos allowed for Ottoman soldiers to overwhelm the fortifications and gain access to the inner wall. According to tradition, Constantine XI was killed in the final assault of the city. On the same day, a small band of Ottoman soldiers were able to slip through Kerkoporta Gate (location uncertain), while the Janissaries breached the walls around the Gates of St. Romanos and Charisius. Mehmet II made his triumphal entry into Constantinople through the Charisius Gate. Later known as Edirnekapı, this gate was extensively restored by Bayezid II and became a ceremonial entrance for Ottoman sultans. Other sections of the Theodosian Walls were extensively damaged during the siege, which were subsequently restored by the Ottomans.

In 1458, a citadel known as Yedikule (“Seven Towers”) was constructed behind the Golden Gate. As earthquakes repeatedly damaged the city walls, multiple repairs were made during the Ottoman era. Towards the end of the Ottoman era, they were increasingly neglected and sections of the walls collapsed. In 1886, the Xylokerkos Gate, which had been bricked up during the Byzantine era, was opened up again, after which it became known Belgradkapı (“Belgrade Gate”). At the beginning of the 20th century, plans to demolish the Theodosian Walls in order to modernize the city following European models were rejected due in part to the intervention of Turkish and foreign archaeologists. A section of the walls near the Sea of Marmara was demolished during the construction of the railway built in 1871. In the 1950s, other sections of the walls were demolished to build major roads like Millet Caddesi and Vatan Caddesi. The walls, though in ruins in many sections, were included in the UNESCO World Heritage Sites of Istanbul in 1985.
The remains of several sections of walls are located in the region of Blachernai, which are often claimed to represent walls dating to the Theodosian period or even before. The original line of the Theodosian land wall in the area has been debated; it is only certain that it once ran behind the Church of Theotokos Blachernai. A section of limestone wall, southeast of the present Church of St. Demetrios Kananou near the Golden Horn, is the most likely candidate for a surviving section of the Theodosian Walls. As it was incorporated into the sea wall, it might have survived the destruction of the Theodosian land wall. 

See also Golden Gate, Walls of Blachernai