The Walls of Blachernai, also known as the Pteron, consist of a series of walls built after the 5th century Theodosian walls were completed. They were built in order to bring forward the walls from the original line of walls to protect the Church of Theotokos Blachernai and later to protect the Palace of Blachernai. These walls, extending from the Palace of the Porphyrogenitus (Tekfur Sarayı) to the Golden Horn, consist of the Wall of Heraclius, the Wall of Leo, and the Wall of Manuel Komnenos.

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. The Avars, after failing to breach the walls, laid waste to the region of Blachernai, destroying the churches outside the city walls, including the Church of SS. Cosmas and Damian. The Church of the Theotokos Blachernai, which was the most famous Marian shrine of Constantinople at the time, survived the siege, and was regarded as being miraculously spared. Its famous icon of the Theotokos also was seen as playing a role in protecting the city. In order to protect this important church, Heraclius extended the walls to enclose the church. His fortifications consisted of a single wall around 500 meters long that had approximately 13 rectangular towers.

Leo V (813-820) erected another wall in front of the Wall of Heraclius following the Bulgarian siege of Constantinople in 813. The Bulgarian khan Krum (c. 802-814), after killing Emperor Nikephoros I (802-811) in battle, captured Adrianople and devastated the environs of Constantinople in 813 at the beginning of the reign of Leo V (813-820). Tower B19, known as the Tower of St. Nicholas, seems to have derived its name from a church dedicated to St. Nicholas, which was once located in the vicinity. A holy spring (hagiasma) dedicated to St. Nicholas still exists between the walls of Leo and Heraclius. Work on these walls continued during the reigns of Michael II (820-829) and Theophilus (829-842). During this time, the Wall of Heraclius was also strengthened, with the addition of three large hexagonal towers (Towers B 15, 16, and 18) and a staircase. An inscription fragment of Theophilus can also be seen on the so-called “Tower of Anemas”, which is adjacent to the “Tower of Isaac Angelos” built later. Around the same time, Xyloporta (“Wooden Gate”), which survived until 1868, was built. These two walls, which had become something like a citadel, were known as the Brachiolion (“Bracelet”) of Blachernai. This section of the walls was particularly vulnerable, as it was often a point of attack multiple times through history. Even the Crusaders were able to scale the Wall of Heraclius in 1203, but were later forced to retreat.

During the reign of Manuel I Komnenos (1143-1180), a new wall was built to better secure the Palace of Blachernai, as it had become the customary imperial residence starting in the reign of Alexios Komnenos (1081-1118). While the palace itself was strongly fortified as to resemble a castle, this wall was built 100-120 meters west of the older wall. It runs between the Prison of Anemas and Palace of the Porphyrogenitus, and had three gates (including the Gyrolimne Gate and Kaligaria Gate) and 13 towers. While it is unprotected by a moat, its wall is more massive, reinforced by relieving arches, and is flanked by stronger towers that are closer together. The first six towers (Towers B1-B6) along the Wall of Manuel Komnenos are alternately round and octagonal, while Tower B7 and B8 are octagonal, and Towers B8-B13 are square. The so-called “Prison of Anemas”, built in the reign of Alexios I, consists of a series of 14 chambers behind the buttress wall. It is extremely difficult to access, in part because it was significantly altered over time. A large residential tower (Tower B14) was built from the remains of older structures in the area in 1186/87 by Isaac Angelos (1185-1195, 1203-1204) in front of the “Prison of Anemas”. Blachernai was an important location for several events in the Fourth Crusade and its eventual sack of Constantinople. A Crusader army first camped at the walls of the palace and attempted an assault there in 1203. The Crusaders failed again in an attempt to attack the palace’s walls in 1204. Work on the Walls of Blachernai took place at an indeterminate time during the Palaeologan Era. In 1441, John VIII Palaiologos (1425-1448) repaired battlements and towers on the Wall of Blachernai as part of the general repair work. During the Siege of Constantinople in 1453, the Ottomans bombarded the Walls of Blachernai with canon, damaging the Tower of Isaac Angelos and the Kaligaria Gate (Eğri Kapı).

The remains of several sections of walls are located in the region of Blachernai. It is often claimed that these represent walls dating to the Theodosian period or even before, though this is far from certain. Suggesting it belongs to a pre-Theodosian period involved connection Blachernai with Region XIV in the Notitia Urbis Constantinopolitanae, which lists it having a palace and its own walls. The remains of these walls, then, has often been associated with the walls of Region XIV. However there are also reasons to doubt its identification with Blachernai, in part because it is described as having a “double line of walls”, thus is seeming disconnected with the Theodosian Walls that ran near Blachernai. The original line of the Theodosian land wall in the area has also 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. As it was incorporated into the sea wall, it might have survived the destruction of the Theodosian land wall. If this is the case, it could mark the beginning of the Wall of Heraclius as well.  There are remains of a wall behind Tower B13, which runs perpendicular to the Wall of Manuel Komnenos. It is possible that this marks the original line of the Wall of Heraclius. The Mumhane Wall is located further south and seems to align with the wall behind Tower B13. It has the remains of two towers. As masonry shares features with the Theodosian land walls, this wall might have been built using material from the Theodosian land walls that once passed through the region of Blachernai. The Church of Panagia tes Soudas might also confirm the line of this wall, as its epithet ‘tes Soudas’ means “in the moat”. Between Tower B13 and the Mumhane Wall, there are also traces of another wall. It seems to have originally followed a line to the wall of the courtyard of the Palace of the Porphyrogenitus. The remains of other walls, including the terrace walls of the Palace of Blachernai, have also survived in the area. Two cisterns, perhaps dating to the Late Byzantine era, are also located at the “Prison of Anemas”. More recently, the remains of a Byzantine church were unearthed in connection with restoration work on the Prison of Anemas.