"Castra Martis" fortress is situated in the center of town Kula in a park area, on some 32 km southwestwards from town of Vidin and on 13 km from the border of Republic of Bulgaria with Serbia, where is located Vrashka Chuka border check point.
The archaeological studies have shown that before the construction of the fortress along the slope of Voinishka River was a small Thracian-Roman settlement, where life began as early as the first millennium BC and continued into the early centuries of the Roman Empire. The fortress dates back to approximately the end of the 3rd century and played an important role in the defense of these lands in the 4th – 6th century. The life of this place and the fortress are connected with the history of our land during the Roman and early Byzantine era (1st – 4th century). After their conquest by the Romans in the beginning of the 1st century the lands between the Danube and the Balkan Mountains were included in Moesia. The establishment of the so-called Danube limes (border) started, consisting of military camps, settlements, watchtowers and other facilities along the southern bank of the river. With the conquest of Dacia (today Romania) boundary moved to the north. Favorable conditions for the development of agriculture were created, as well for the crafts and trade, and for the urbanization of Moesia. The major city in the region between the rivers Tsibar and Timok was the town Ratsiaria, today Archar. In Vidin is the fortress of Bononia.
At the end of the III century Roman rule in Dacia was rejected. These events and the invasions of the Goths forced the central government to take decisive measures to consolidate the Balkan possessions. In connection with the administrative reform was created Riverside Dacia with main city Ratsiaria. At the end of 3rd and early 4th century has recovered strongly Danube limes, and on important strategic points inside fortresses were constructed new fortresses. One of these newly constructed forts in Riverside Dacia is "Castra Martis" (Mars Fortress). It held a key position in the westernmost mountain pass Vrashka and guarded the important Roman road from Bononia to Singidunum (Belgrade).
Castra Martis played an important role in the defense of these lands in the 4th – 6th century, for which we have information about ancient authors. In 377 through it passed with his squad Gratsiyan Emperor on his way to Thrace. In 408 it was captured for a short time by Huns' leader Uldis and others.
The fate of the fortress reflects the collapse of the slave society and destruction of the Roman Empire. It suffered destruction during the Gothic invasions at the end of 4th century, but most suffered from the Huns invasions in the first half of the 5th century, starting with its conquest by Uldis in 408, for which there is historical evidence. According to the Byzantine chronicler Procopius' Castra Martis "is one of the main strongholds recovered at Justinian the Great (527-565). The efforts of the Eastern Roman Empire to retain power in Danube-around lands were in vain. In the 6th century the Avars and Slavs invaded it. "Castra Martis" was destroyed during the invasions of the Avars from 586 to 587, after the interruption of life here for centuries the fort was partially restored and used again in 13-14th century in defense of the Vidin feudal kingdom.
Archaeological studies have shown that the fortress consists of two parts, a small square fort-quadriburgius and a castle, located south of it. Both parts are designed and implemented simultaneously. Originally it was erected the quadriburgius –at the end of 3rd , early 4th century. This type of fortifications in the border regions of the Roman Empire are associated with fortification activities of Emperor Diocletian. Quadriburgius represents a square fort, with size of 40 x 40 meters, at its corners are raising powerful round towers with a diameter of 12.5 m. It was a seat of the town-major of the fortress.
The castle has the shape of an irregular quadrangle, defended by 7 polygonal towers. It occupies an area of 15.5 acres. In the middle of the central building of the fort there is a courtyard with a well, covered with large square bricks. Around the courtyard there are two-storey premises. North from the yard there is a large room which is used to have special functions. There probably command staff gathered and stored battle flags and medals of the army unit camping in the fortress. East and west from the main building there are 5 massive pylons which carried the construction of the two-storey rooms to the fortified walls, which were used in the down floor for stores and the upper for barracks. The towers were of three floors connected by wooden stairs. They were covered with tiles. For their defense were used arched windows.
South from the quadriburgiuswas situated the camp of the army, camping in the fort. At the second quarter of 4th c probably during the reign of Constantine the Great /306- 337/ and his sons it was surrounded by thick- 4, 30 m fortifying walls.
Quadriburgiuswas better preserved and completely revealed. Fortress walls were made of stone and three-row brick shelterbelts having thickness of 2, 20-m. there are preserved on average of a height of 2 m above the floor, and the south tower rises to 16, 30 meters. It was only accessible from the south, where it was located its gate, closed with double doors. Later, probably at the end of 4th century, the protection of the gate was reinforced by the construction of another narrower wall lying at 3, 30 meters from the first. Thus were formed the sentinels rooms and a over-door tower. Northwest of it were discovered the foundations of a Roman bath.
Through the gate you enter in an open courtyard, where is located a rail staircase. It has led to a battle platform of the walls that rise 10 meters in height and were finished with narrow loopholes. Through the same stairs one could entered to the second floor of the south tower and the rooms to the east wall.
The central building of the fort was built on the north wall. In the middle there is a courtyard with a well, which was paved with large square bricks. Around the courtyard there are two-storey rooms with different functions, serving the commandant of the fortress.
The rich living material and the diversity of the tools found with the excavations of the fort shows that the inhabitants of the fortress had additional agriculture farm in the area.
Next to the ruins in a small building there is a museum collection of artifacts found during the excavations that reveal the material culture of the residents. Materials from Thracian-Roman settlement before the construction of the fortress are scarce. Hand made pottery, marble columns, bronze statuette of Venus portrait head from 3rd century, altar dedicated to Jupiter, and more are presented. With more materials, plans and maps is illustrated the life of the late antique fortress. Below there is a variety of tools: agricultural, cattle, bricklaying, mason and others, used in subsidiary farm of military unit. For the life of residents testify clay lamps and various shaped containers, covered with green and brown glaze. Below are the bronze brooches and coins from 4th-5th century. The points of spears and arrows bear witness to the turbulent events that fortress had experienced, played an important role in the defense of these lands in the 4th-5th century.