The north-western corner of Bulgaria has always been a frontier. The Danube runs along it, wide and slow, and whoever controlled the river controlled the movement of armies, goods and people between the plains to the north and the Balkan interior. At Vidin, Baba Vida rises directly from the riverbank, a compact quadrangle of walls, towers and a moat that grew on the site of an earlier Roman settlement. It is the best-preserved medieval fortress in the country, and one of the few where a visitor can still walk the wall-walks, climb the towers and look out over the Danube exactly as a garrison once did; its courtyard is used today for open-air performances. A short distance inland, at Kula, the ruins of Castra Martis mark a late Roman fort guarding the road network behind the river. Less complete but earlier, it supplies the missing chapter: the Roman organisation of the frontier on which the medieval defences were later grafted. Filmed in 4K, with aerial views along the Danube and inside both enclosures, the programme suits strands on history, military architecture and archaeology.
