The city-mound of Roman Poetovio looms over the later Slovenian city of Ptuj. All of its buildings, including the fortified base camp of the Thirteenth Legion, have been levelled. Yet its elevation remains the same as that of Ptuj’s medieval castle which survives intact. When I climbed the mound in the early July of 2024, there were fragments of marble bas-reliefs here and there, and the footings of legionary fortifications, all the way to the top – where a fresh excavation was laying bare the pebble foundations of a sanctuary wall. From here there was a long view of the course of the Drava, a river that flows from the Alps into the Danube. Here it is wide and fast-flowing, but not far downstream there is – or was – a strategic crossing point, which is why the Romans dug in right here, establishing a regional capital with a population of 40,000. (Today’s inhabitants number only 23,000.)

The medieval and modern city developed alongside the ruins of Poetovio, and hardly encroached on the far bank until the middle of the twentieth century. Yet it is on this western bank that some of the most significant remains have been found, revealing that many of the inhabitants – of all ranks – were adherents to the cult of Mithras. Twenty-two Mithraic shrines have been discovered so far, and there may be others out there. Mithras was the favoured deity of army personnel, and there are good reasons why. Every legionary was contractually bound to twenty years of service. Few survived that ...