The Fortification System of Diocletianopolis during Late Antiquity

The Fortification System of Diocletianopolis during Late Antiquity
National Archaeological Institute with Museum"                                                             DOI                                                 
Bulgarian Academy of Sciences   
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Abstract:  Diocletianopolis is one of the main cities of the late antique province of Thrace and, at the same time, the center with the best-preserved fortification system from the 4th–6th century period in the eastern part of the Balkan Peninsula. The fortification of the city consists of all the main elements of fortress construction during the period, and it is a classic example of this type of structure in the region. The defense of the city was developed over several construction periods, defined by the large waves of enemy invasions of foreign peoples from the north.

Keywords: Fortification, Thrace, Diocletianopolis


Diocletianopolis is the third most important city in the province of Thrace[1]. Its urban center is located below the central part of the present-day city of Hisarya, 42 km north of Philippopolis (present-day Plovdiv). The fortress of Diocletianopolis is the best-preserved fortress of the late antique province of Thrace, providing a significant opportunity for analysis and commentary. Almost the entire fortification system of the city has survived to the present day. Scholars brought attention to the site during numerous studies in the 20th century. The information obtained during the years of excavations and examination are summarized in volume I of the archaeological study dedicated to the city[2]. In this research, the data concerning the fortification of Diocletianopolis are presented in detail; the information is correct, although some of the author's conclusions are subject to some criticism[3]. Among the still controversial issues is the site’s initial date.

At the time of the Principate (1st–3rd centuries), the settlement around today's Hisar mineral springs was not fortified. Although scholars have speculated on the existence of defensive structures from the time of Emperor Marcus Aurelius[4], there is no evidence of their existence. The construction of the fortress wall should be attributed to the period after the Gothic invasions and should most likely be connected to the reign of Emperor Diocletian whose building projects gave the settlement its name and city status[5]. The fortress of Diocletianopolis, built at that time, maintained its structure through all the later alterations. It encloses a space in the shape of an irregular quadrangle, with several turns along the course of the western, southern, and eastern curtain walls. The total length of the wall is 2,327 m and encloses an area of ​​about 30 ha. In the superstructure, the wall has a thickness of between 2.60 and 3.00 m. Along its entire length, the curtain is interrupted by 44 towers and 4 gates – one on each of the walls. The individual segments of the curtain range in length from 38 m to 62 m. In height, it is preserved up to 13 m at the western gate, and at the southern end of the wall it has been destroyed to the level of the banquette. It was built in opus mixtum, with up to 11 two-row, three-row, and four-row brick belts[6].

During the first construction period, Diocletianopolis had three main city gates situated on the western, eastern, and northern walls. The western gate is the best-preserved part of the fortress; it is 13.20 m high, a result of the fact that it is in a slight depression of the terrain. The structure is a vaulted passage, 4.20 m wide and 9.50 m high. The plinth of the gate is built of reused stone blocks between which are two commemorative inscriptions from the time of emperor Alexander Severus; these can be taken as the terminus post quem for the construction of the gate and the fortification of Diocletianopolis as a whole[7].

In addition to the four towers at the corners of the fortress, the wall of Diocletianopolis is divided by another 40 intermediate towers with a quadrangular plan. Both western corner towers are rectangular in plan; the north-east corner tower of the fort, however, is octagonal and rests on a circular base. The southeast corner tower is fan shaped. All towers protrude in front of the curtains while remaining structurally connected to them. The towers are built in mixed masonry, except for the outer corners and the entrances to the fortress which are made only of bricks.

During the initial construction of the wall, openings were left for six porters – two in each of the southern, western, and northern curtains. Their structure is very similar, a sign that they were constructed around the same time[8].

Stairs located in the separate sectors of the fortress provide access to the platform at the top of the fortress walls. They are single-armed or double-armed, attached longitudinally to the curtain on its inner side[9].

The fortification had another important element. Researchers recorded a defensive moat in front of the entire southern and eastern walls, as well as in front of the eastern half of the northern wall. It was located 10–15 m from the curtain and had a width of between 15 and 20 m. Its depth reached 4 m below the level of the field[10].

By analogy with the other fortified centers in Thrace, the next construction period for the fortification of Diocletianopolis dates back to the Gothic invasions in the last quarter of the 4th century[11]. The building activity of this time included the repair of several ruined sectors of the wall, as well as repair work on the collapsed eastern gate. It also included the construction of the new southern gate, today widely known for its scale and the exotic shape of its ruined arch. All other elements of the fortification system were restored to their original form or remained intact after the attacks.

The structure of the southern gate was erected approximately in the middle of the southern fortress wall, on the foundations of the fully destroyed curtain. Its location is aligned with the opposite northern gate, both located in the same axis. Its structure is similar to the other city gates, but with some significantly more complex details. Its passage is longitudinally vaulted. The front of the structure, in the space between the two forward-projecting towers, was also covered by a vault which was supported by two extensions of the walls of the towers specifically build for this purpose[12]. A smaller arch, preserved above the vaulted entrance, indicates the existence of a third defensive tower located above the passage. It likely reached a height of 3.50 m, topped with a battle platform[13]. This gate is decorated with a horizontal brick cornice running along the facade above the arch on the inside of the gate. It supported a triangular pediment, the arms of which are built of four rows of bricks. This plastic decoration is completely defaced by crumbling bricks[14].

At the end of the 4th – the beginning of the 5th century, several barracks were built, attached to the inner side of the southern and eastern walls.

The third construction period in the fortification system is likely related to the time after the devastation caused by the Hunnic invasions in the middle of the 5th century. During this time, the northern and eastern gates were destroyed but immediately restored to their previous form[15].

Some of the barracks also appear to have been abandoned at this time – for example, at the western end of the southern wall, on top of the remains of one of the barracks a Christian Basilica was then built.

The proteichisma built in front of the northern fortress wall should also be included in the structures of the 5th century. It is 500 m long and is mostly preserved up to 2 m high. It is straight along its entire length, with a wall width of 3.00 m. Its height during its construction is calculated to be 6 m, and its distance from the city curtain is 10.50 m. The main entrance facility in the protechisma is located directly opposite the northern gate of the fortress. It was also equipped with seven postern gates along its route.

The proteichisma must be dated to or after the middle of the 5th century. K. Madzharov associates it to a separate, fourth construction period before the renovations in the northern and eastern gates. At the same time, the building of the proteichisma was generally dated by him to the time of Emperor Theodosius II (402-450), by analogy with the synchronous proteichisma in Constantinople. Madzharov's arguments are unconvincing, as are the analogies with similar structures from present-day Bulgaria – all dated to a later period[16]. It should be taken into account the fact that the proteichisma and, more specifically, its gate does not bear traces of destruction and alterations. This, in light of the demolition of Diocletianopolis’ northern gate by the Huns, comes to show that the assumption of its earlier construction is unfounded. It appears that its construction took place at a later date. Taking this into account, we can better understand the use of a material previously unused in the city, which was found even in the remodeling of the gates and the curtain damaged in the Hunnic raids. Such an opinion is advocated by D. Ovcharov, who dates the proteichisma to the middle of the 5th century at the earliest[17].

In the 6th century, some minor repairs and changes were made to the fortification system. In the south-west corner of the fortress, researchers registered several patches in the curtain and the wall of the south-west corner tower; these disrupted the symmetrical brick belts. Some of the postern gates were blocked off also during this time[18].

The separate construction periods of the fortification system of Diocletianopolis within the framework of late antiquity are clearly discernible. Like Augusta Trajana, there is no abandonment of the late Roman fortification and no reduction of the protected area. At the same time, however, the fortification of the city in late antiquity did not supersede structures from the Roman period. Thus, at the beginning of the 4th century, Diocletianopolis had an excellent newly built fortification system fitting the city’s scale and importance. It should also be noted that the southern fortress wall acquired an important role at the beginning of the early Byzantine period, with the construction of the main representative gate of the city and a large number of barrack buildings in this part of the settlement.


[1] Velkov, 1959, 106

[2] Madzharov, 1993

[3] Boyadzhiev, 1996, 43-44

[4] Boyadzhiev, 1967, 109; Boyadzhiev, 1972, 189-191; Boyadzhiev, 2000а, 103

[5] Madzharov, 1993, 61

[6] Madzharov, 1993, 24-30

[7] Madzharov, 1993, 42-43, 61

[8] Madzharov, 1993, 49-52

[9] Madzharov, 1993, 30-33

[10] Madzharov, 1993, 58

[11] Madzharov, 1993, 61-62

[12] Madzharov, 1993, 43-45

[13] Boyadzhiev, 1972, 172-176

[14] Madzharov, 1993, 46

[15] Madzharov, 1993, 48-49

[16] Madzharov, 1993, 55, 62

[17] Ovcharov, 1973, 17

[18] Madzharov, 1993, 50-51



Бояджиев Ст. (1967) Нови данни за Хисарските стени, Известия на Археологическия Институт, XXX, 101-112/ Boyadzhiev St. (1967) Novi danni za Hisarskite steni, Izvestiya na Arheologicheskiya Institut XXX, 101-112

                      (1972) Нови проучвания върху портите на римския град при Хисар, Известия на Секцията по Теория и История на Градоустройството и Архитектурата, XXIV, 165-191 / Boyadzhiev St. (1972) Izvestiya na Sectsiyata po Teoria i Istoria na Gradoustroystvoto I Arhitekturata XXIV, 165-191

Маджаров К. (1993) Диоклецианопол, т. I, София / Madzharov K (1993) Dioklecianopol, vol. I. Sofia

Овчаров Д. (1973) Протейхизмата в системата на ранновизатийските укрепления по нашите земи, Археология, 4, 11-23 / Ovcharov D. (1973) Proteyhizmata v sistemata na rannovizantiyskite ukrepleniya po nashite zemi, Arheologia 4, 11-23


This page is part of the project LABedia: Еncyclopedia of Late Antique Balkans, 4th-5th c.,
financed by the National Science Fund, contract КП-06-Н30/6, 13.12.2018