The ruins of the Ancient Roman fortress Castra Martis (meaning “Mars’s Fortresses" in Latin) are located in the center of the town of Kula, Vidin District, in Northwest Bulgaria. The name of the town Kula (meaning “tower" in Bulgarian) is based on the sole surviving 16-meter tall fortress tower of the ruins of Castra Martis.
Castra Martis overlooks the Voynishka River, and actually consists of two late Roman fortresses located next to one another – one from the end of the 3rd-beginning of the 4th century AD, and another from the second quarter of the 4th century AD.
Archaeological excavations have revealed that the construction of the fortress was preceded by the existence of a small Ancient Thracian settlement dating back to the 1st millennium BC, and surviving into the early period of the Roman Empire.
After they conquered the territory of today’s Northern Bulgaria, i.e. the southern bank of the Lower Danube at the beginning of the 1st century AD, the Romans erected the so called Limes Moesiae, the frontier fortifications of the Roman Empire along the Lower Danube. In the 2nd century AD, they conquered the Dacians and the frontier moved north of the Danube. However, at the end of the 3rd century AD, the province of Dacia was lost, and the Limes Moesiae south of the Danube was restored to protect the Empire.
In 271 AD, Roman Emperor Aurelian (r. 270-275 AD) transformed the province of Moesia Superior into the province of Dacia Aureliana with its capital at Serdica (today’s Sofia), after vacating Dacia Traiana beyond the Danube. Around 283 AD, Dacia Aureliana was divided into two provinces, Dacia Mediterranea, with its capital at Serdica, and Dacia Ripensis (“Dacia from the banks of the Danube") with its capital at Ratiaria (Colonia Ulpia Ratiaria). Castra Martis was located in the province of Dacia Ripensis.
The fortress of Castra Martis was part of the restored Limes Moesiae. It is believed that the first fort of Castra Martis was built by Roman Emperor Diocletian (r. 284-305 AD) around the turn of the 4th century. It protected a road from Bononia (today’s Vidin) to Singidunum (today’s capital of Serbia Belgrade) going through the Vrasha Chuka Pass, which is one of the westernmost passes in the Balkan Mountains.
The fortress of Castra Martis was mentioned in ancient sources in 377 AD, when Roman Emperor Gratian (r. 375-383 AD) stopped there briefly with his troops on his way to the province of Thrace. Castra Martis was damaged during the Gothic invasions at the end of the4th century AD. In 408 AD, the fortress was taken by the Huns under their chief Uldin during a war against the Eastern Roman Empire (Byzantium). In the 5th century AD, Byzantine historian Procopius of Caesarea mentioned Castra Martis as one of the Roman fortresses rebuilt by Byzantine Emperor Justinian I the Great (r. 527-565 AD). The fortress was destroyed during the barbarian invasions of Avars and Slavs in Byzantium in 586-587 AD.
In the 13th-14th century, during the Second Bulgarian Empire (1185-1396 AD), Castra Martis was partly rebuilt, and was used for the defense of the Vidin Tsardom against the invading Ottoman Turks at the end of 14th century.
The fortress of Castra Martis consists of two major fortifications – a small older quadriburg, and a larger castra (fortress) located to the south of the quadriburg. The quadriburg dates back to the end of the 3rd – the beginning of the 4th century AD. Numerous similar quadriburgs were built during the reign of Emperor Diocletian in the frontier regions of the Roman Empire
The quadriburg at Castra Martis was a rectangular fort which was 40 meters long and 40 meters wide. It had large fortress towers at its corners, each with a diameter of 12.5. The quadriburg has been well preserved, and fully excavated. Its walls were built using stone and bricks, and were about 2.2 meters wide. The ruins of the fortress wall have been preserved up to an average height of 2 meters. However, the southeastern tower has been preserved up to a height of 16.3 meters; it is this tower that has given the name of the modern-day Bulgarian town of Kula (“Tower"). The quadriburg had a single gate located at its southern wall. At the end of the 4th century, the gate was strengthened with a second wall located 3.3 meters away from the original wall.
Inside the quadriburg, the main building was located closer to the northern fortress wall. In the middle of the fortress, there was a yard lined with two-story buildings, with a well.
The Roman military camp at Castra Martis was located to the south of the quadriburg. In the second quarter of the 4th century AD, possibly during the reign of Roman Emperor Constantine I the Great (r. 306-337 AD), it was turned into a fortress with walls that were 4.3 meters wide. It has a rectangular shape with 7 polygonal towers, and a total area of 15.5 decares (almost 4 acres).
To the northwest of the fortress, the archaeologists have found the foundations of Ancient Roman thermae (public baths) which are construed as evidence that there was civilian population living outside the fortress of Castra Martis.
Castra Martis was first explored and identified in the 1870s by Austro-Hungarian geographer and archaeologist Felix Kanitz. It was excavated by Bulgarian archaeologists in the 1970s. The fortress of Castra Martis in the town of Kula was declared an architectural monument by the Bulgarian authorities in 1965.
Today its ruins are part of the Castra Martis Historical Park which also features a museum collection with artifacts discovered during the archaeological excavations of the Roman fortress.