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The Fortification System of Augusta Traiana during Late Antiquity
|National Archaeological Institute with Museum"||DOI|
|Bulgarian Academy of Sciences|
Abstract: Augusta Traiana is the second most significant city of the province of Thrace and an important fortified center. The city possesses a complicated fortification system that makes use of non-traditional solutions, including the most current elements of poliorcetics from Late Antiquity. Augusta Traiana follows the framework of construction stages typical for the region; its fortification consists of a unified complex of a double fortress wall and towers of different structures to prevent massive enemy attacks.
Keywords: Fortification, Thrace, Augusta Traiana
The fortification system of the urban centre of Augusta Traiana is the largest and most complex system found in the province of Thrace. The city was founded at the beginning of the 2nd century AD at the site of the older Thracian settlement of Beroe. During the first period of its existence, the Roman Augusta Traiana remained unfortified until the end of the third quarter of the 2nd century. During the time of Emperor Marcus Aurelius, probably as a result of the Marcomanni raids, the first fortress wall was erected. This happened around 172 AD when the rest of the Roman centers in Thrace were also fortified. During the 3rd and 4th centuries, the great Gothic attacks of 251 and 375–376 AD affected the city. The second Gothic wave caused significant destruction in the fortification system, which necessitated the rebuilding of the fortress wall; the changes were made according to the development of poliorcetics at the time (end of 4th – beginning of 5th century). The third construction period for the city fortification is from the end of the 5th to the beginning of the 6th century. This period is characterized by the restoration of the fortification systems throughout Thrace after the destructive Hunnic invasions. The late antique walls were restored and used during the medieval and Ottoman periods of today’s city of Stara Zagora. The wall’s height remained until the end of the 19th century; this can be seen in the precisely prepared plan of the Czech scholar L. Bayer from 1879, immediately after the Liberation of Bulgaria. The accuracy of the "Bayer" plan is confirmed by all the studies carried out in the 20th century. In modern studies, it is used as a basis for the layout of the ancient city, with a few minor corrections added following a number of rescue archaeological excavations.
The protected area of Augusta Traiana was defined in the Roman period and remained unchanged for centuries. The fortress has the shape of an irregular polygon, although it is located on a flat plain. Despite the approximate orientation towards the cardinal directions, the irregular plan of the wall raises the question of why, and on what technical grounds this decision was made for its construction. The most evident reason is the need for maximum stability for the foundations of the fortress, which is hampered by a large groundwater bed. According to another opinion, the walls’ plan is aligned with the beds of two small rivers contemporary to the construction.
The area along the southern and western route of the curtain has been studied in most detail. The fortification wall is built of rubble stones with a mortar joint. In the principal study on Augusta Traiana’s Roman fortification system, the author defines the wall construction technique as Opus vittatum mixtum.
In terms of the superstructure, the wall is 2 m wide and is located on the outer part of the plinth. Pilasters set along its length form niches in the wall; in different sectors, the distance between the pilasters changes, and on the southern route it reaches up to 8 m. Two gates of the fortress, built with identical masonry and construction, have been revealed on the southern and western walls. The western gate is believed to be the main city gate. Its location in the southwest corner of the fortress raises doubts on this issue, although, according to some researchers, the discovery of the so-called forum complex in the immediate vicinity of the western gate provides sufficient evidence for its importance. The gate contains a gate tower with a double closure – external and internal cataract – which form a propugnaculum with dimensions of 6.50 x 8.00 m. It was built in the opus quadratum technique. Two towers from the Roman period are registered in the fortress wall; both are rectangular in plan, set into the fortress 1/3 of their length. They were found in the western and northeastern course of the wall, respectively. In front of the western gate, an 8 m wide defense moat was uncovered. In other sectors, the moat was not registered.
The fortification was reconstructed in the last quarter of the 4th century; in some places, the wall was only altered or upgraded, while in other places it was rebuilt. Along its entire route, the distance between the pilasters is filled with mixed masonry. This way, a more fortified wall is formed between 3.30 and 3.65 m wide. In the places where the wall was completely destroyed, it was rebuilt anew with the new thickness, and its inner and outer faces were revetted with opus mixtum masonry. The inner space of the quadrangular tower at the north-eastern end of the fortress was filled with rubble on top of its preserved ground floor. In the sectors where it was necessary to rebuild the fortification from the ground up, the plinth was raised in height. Researchers have noted many spolia – reused blocks and architectural details from the ruins of the old fortress wall and buildings.
At the end of the 4th century, the fortress suffered the greatest destruction in the areas around the gates. However, after reconstruction, the entrances generally retained their previous appearance and were rebuilt in the same technique. The fortification system still lacked flanking towers during the second building period. Tactical changes were also made, including the abandonment of the internal cataract. Reconstructions have also been carried out in the outer part of the gate. Here, a double-winged door was mounted at the level of the outer face of the wall. Behind it a cataract closed the entrance, flanked by two rectangular pylons jutting out of the wall. There is a lack of data regarding the changes that took place in the construction of the western gate during the first two construction periods. There are two different theories about the evolution of the entrance complex within the technical reconstructions. In general, the structure of the southern gate repeats the plan of the western one.
During the second construction period, numerous towers which did not exist in the Roman fortress were erected along the route of the wall; they are structurally embedded in the newly built wall above the plinth. All of them are rectangular in shape and protrude entirely from the curtain wall. They are built in the opus mixtum technique, with 4 or 5 brick belts; the entrances, however, are built in opus testaceum. Some reused blocks, slabs with inscriptions, and arae are found in the masonry of their plinths.
The most extensive period of restoration in the fortification of Augusta Traiana occurred after the Hunnic destruction. This period is characterized by both the repair of the old fortress wall and, parallel to it, the construction of a new fortress with the proteichisma along its entire length. All studies on Augusta Traiana’s fortress wall state that the newly built proteichisma is a complete duplication of the old wall; although, this has not been attested by archaeological excavations along the entire length of the curtain. The assumption was partially confirmed by the excavations along the western route of the two walls carried out in 2005. In this sector, the outer fortress wall rises immediately next to a significant slope, which could serve as a natural defense. This situation refutes the suggestion that the proteichisma was built only in front of some sectors of the inner wall. It was built in the opus mixtum technique with 4 and 5 rows of bricks belts, along with many reused materials. The curtain is 2.50–2.60 m in thickness and likely up to 8 m in height.
With the construction of the proteichisma, a passage was formed between it and the inner fortress wall; it had a width of between 3.50 and 8.00 m, which was blocked at intervals by the massive towers from the previous periods projecting in front of the inner fortress wall. In addition, the possibility of enemy maneuvering within the passage was hindered by the triangular towers newly added to the outer face of the inner wall. This way, the unity of this space between the two fortress walls is broken. This feature should be attributed to tactical considerations in the construction of the defense system, aimed at cutting off the access of a potentially successful attack through the protechisma to all sectors of the inner fortress wall. At the same time, they ensured entry to separate parts of the passage for the defenders of the fortress by means of postern gates in some of the towers and the inner fortress wall. Scholars have questioned the function of the proteichisma of the outer fortress wall, theorizing that the passage between the two walls was covered, and the resulting galleries – approximately 3.50 x 50 m – were used for barracks.
Researchers have recorded several staircases attached to the inner face of the protechisma. A total of four postern gates were found on the outer fortress wall.
After the destruction of the gates from the end of the 5th to the beginning of the 6th century AD, they were rebuilt in the form of a complex that closed the entire space between the old fortress wall and the proteichisma. Both gates maintain the towers’ height but are different in scale. The tower above the western gate is 18 m long and 9.50 m wide – it probably rose one storey above the wall. One could pass through the facility through four doors – two on the outer wall and two on the inner wall. A propugnaculum with a rectangular shape and a length of 6 m is formed between the two walls. The architect Boyadzhiev maintains the potential presence of a drawbridge over the moat; the basis for his assumption is the exposed masonry on the opposite side of the moat which most likely served as a bridge step. The south gate was rebuilt in the same way, only differing in size. The dimensions of this defensive structure are 11 x 8 m, and the propugnaculum is 4 m long.
To date, a total of four triangular towers have been studied, all dating to the third construction period. Their development is connected with the rebuilding of the Roman wall and with the construction of the proteichisma parallel to it. They are attached to the old curtain and, at the same time, secured by the outer fortress wall. The individual towers show some differences in their arrangement and construction.
Augusta Traiana emerges as the only large city in the diocese of Thrace which did not reduce its protected area during the 5th–6th centuries. Augusta Traiana's fortification system was also used in the early Middle Ages, as evidenced by a building inscription in Greek found in the reconstructed sector of the western fortification system. This inscription dates from the last quarter of the 8th century; it mentions the remodeling of the outer fortress wall, the remains of which are found collapsed today on the slope to the west of the fortress. The destruction dates to the third or fourth quarter of the 8th century.
 Boyadjiev, 2000а, 105
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 Boyadjiev, 2000b, 127.
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 Kaltschev, 1998, Abb.8.
 Dinchev, 2001, 235–237.
 Yankov, Kamisheva, 2006a, 278.
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