Castra Martis

Kula, Bulgaria

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Castra Martis is a late Roman fortress located in the town of Kula in northwestern Bulgaria, built at the end of the 3rd century and the beginning of the 4th century as part of the Roman Empire's defensive system along the Danube frontier.

The fortress protected the important Roman road connecting the Vidin (Boninia) region to Singidunum (modern Belgrade) through the Vrashka Chuka pass in the Balkan Mountains. Castra Martis consists of two main parts: a smaller quadriburgium, or square fortification, and a larger castle to the south, constructed in different periods. The fortress was built on the site of a small Thracian settlement dating back to the 1st millennium BC, showing long-term human habitation.

It was repeatedly damaged by Gothic and Hunnic invasions in the 4th and 5th centuries, later restored by Byzantine emperor Justinian I in the 6th century. During the 13th and 14th centuries, the fortress was partly rebuilt under the Second Bulgarian Empire to defend the Vidin region against Ottoman incursions.

The fortress ruins include a well-preserved tower about 16 meters high, which gave the modern town of Kula its name, meaning “tower” in Bulgarian. Archaeological excavations have revealed Roman thermae and other structures, indicating a small civilian population lived outside the fortifications. Today, Castra Martis is an important historical and architectural monument, reflecting its strategic military role and rich historical layers spanning Roman, Byzantine, Bulgarian, and Ottoman periods.