Fortifications are rarely built from nothing. They are placed where earlier builders had already understood the ground, and north-western Bulgaria offers a clear demonstration of that continuity. At Kula, the ruins of Castra Martis belong to the late Roman organisation of the frontier: a fort commanding the roads that ran behind the Danube, of which a substantial tower and sections of wall survive, enough to show the plan and the building technique of the period. Downstream, at Vidin, Baba Vida represents the medieval answer to the same problem. Set directly on the riverbank, its moat, towers and double enclosure make it the most complete surviving castle in Bulgaria, and one where the whole defensive sequence can still be followed on foot, from the gate to the wall-walks and the chambers in the towers. Seen in succession, the two sites make a single argument about frontiers: the same crossings, roads and sightlines mattered to Rome and to the medieval kingdoms that followed. Filmed in 4K, with aerial photography that reveals both plans in their landscape, the programme suits strands on history, archaeology and military architecture.
